From D&D 3.5 to 5.0(5E) - 18 - Spell Concentration

I'm not sure what the rules were in 3.5, but we always played that you could not cast a second spell while you are still concentrating on the first spell you cast, without giving up the first spell.  But in the 5th edition it is clear from page 203 that you can cast additional spells while continuing the concentration of the first spell, as long as the subsequent spells do not also require concentration.

Normal activity such as moving and attacking (I think it is funny that attacking is "normal activity") doesn't break your concentration.  But getting incapacitated does, so does getting killed, and so does casting another spell that requires concentration.  Getting hit in combat may also cause you to lose concentration if the damage is severe enough.

From D&D 3.5 to 5.0(5E) - 17 - Preparing Your Wizard Spells

In my last post I was careful to say you have a limited number of spells until you rest again, instead of for the day, because in the 5th edition of D&D you can recover some spell slots during the day without an 8 hour overnight rest.  Under the ability known as Arcane Recovery, a wizard can choose to reclaim some spell slots after a short rest.  You can recover up to half your wizard levels in slots, rounded up.  So our 3rd level wizard could recover 1.5 spell levels worth of slots (round that up to 2).  The 3rd level wizard could choose to recover 2 1st level slots, or 1 2nd level slot.

Spell casters of many varieties start with a few cantrips, and in the 5th edition you don't have to keep track of how many cantrips you have cast because you can cast them every round all day long.  Cantrips are embedded in the caster's mind and don't require a spell slot to cast.  You can cast them at will.  No preparation is required.

From D&D 3.5 to 5.0(5E) - 16 - Spell Slots for Wizards

The big change for wizards is the use of spell slots.  Just as in all previous editions of D&D, wizards will learn spells and record them in their spell books, but each day the wizard will only be able to prepare a few of those spells.  The number of spells that a wizard can prepare is the sum of his spell level and his intelligence ability bonus.  So a 3rd level wizard with an Intelligence of 18 could prepare 7 spells a day, 3 for being 3rd level and an additional 4 due to his +4 Intelligence modifier. 

So after your wizard has reviewed all his spells in his spell book and selected the spells he wants to prepare he is ready to go.  A wizard would never prepare a single spell more than once.  In 3.5, your wizard might memorize three magic missile spells before adventuring, but in the 5th edition the wizard can prepare magic missile once, but still have the flexibility to cast it zero or many times during the day. 
A 3rd level wizard gets 6 spell slots as shown on the table on page 113.  He gets 4 1st level slots and 2 2nd level slots.  This means that he could cast prepare many different 1st level spells (up to 7), but he could still cast magic missile 6 times, once from each slot.  When a wizard casts a spell, the cast spell essentially fills one of his available spell slots.  And notice that the wizard could cast 6 magic missiles (a 1st level spell) even though he only has 4 1st level spell slots.  That is because you can always use a slot from a higher level to cast a lower level spell, but you can't use lower level slots to cast a higher level spell.  Our 3rd level wizard could cast, at most, 2 2nd level spells before resting.
Some spells, like magic missile, have greater affect when cast from a higher level spell slot.  When casting magic missile from a 1st level spell slot you get three missiles, but when you cast it from a 2nd level spell slot you get four missiles.  When you cast fireball from a 3rd level spell slot you do 8d6 damage, but when you cast it from a 4th level spell slot you do 9d6 damage.  Notice that, unlike the 3.5 edition, fireball damage is not dependent on caster level, only upon the spell slot from which it is cast.
Check out my next post for more changes for wizards.

From D&D 3.5 to 5.0(5E) - 15 - Stuff That Stays the Same

Instead of listing something that changed from 3.5 to 5.0, here is a list of some things that stayed the same:

• For the most part, we still roll similar die for each class for hit points.
• Rounds still last six seconds.
• Initiative is still the same.
• In combat you generally can perform one move plus one action.
• We have the same six ability scores and the same modifiers based on those scores (a score of 12 is +1, 18 is +4, etc.)
• Critical hits still cause twice the normal damage, though rogues now get to double their sneak attack damage, and your strength modifier is only applied once to the total damage.
• We still have slashing, piercing, and bludgeoning weapons.

From D&D 3.5 to 5.0(5E) - 14 - Ability Score Max is 20

You can't increase your ability scores above 20.  This is mentioned several places, including on page 15 and again in sections on classes such as "Ability Score Improvement" on page 49 for Barbarians.

We will still be able to use magic to boost our ability scores though.  And I would be surprised if future content for epic characters does not allow it.
Also, don't be caught of guard.  The ability score limit of 20 applies to characters, not to monsters and divine beings.

From D&D 3.5 to 5.0(5E) - 13 - Saving Throws

Your character will be proficient in two of the three saving throws based on your class which means you will have a saving throw bonus based on proficiency on just those two types of saving throws and no bonus on the third type ever based on your level.  Once again this rule is all about simplification.  We don't need to spend any time when we level up to adjust our base saving throw modifiers.  Instead, when we need to make a saving throw, we roll the d20 and simply add our proficiency bonus if the saving throw happens to be one of the two in which we are proficient.

Page 179.

From D&D 3.5 to 5.0(5E) - 12 - Skill Checks, Where Did They Go?

One task in 3.5 that I often felt took too much of my time was assigning skill points to skills when my characters gained a level, especially for my highly intelligent rogue.  But that task is eliminated in the 5th edition of D&D.  We still have skills and they work similarly, but they are much simpler to administer.  By virtue of your background, your class, your race and possibly a feat, your character will have a set of skills they are proficient in.  When using a skill, your character will apply your ability score modifier (STR, DEX, CON, INT, WIS, CHA) to your die roll, and then if your character is proficient with the skill used add your proficiency bonus.

This makes leveling up simpler because there are no skill points to assign, though it does reduce some of the flexibility we had in 3.5 for configuring our characters.  I don't know if I like this game rule change yet or not, but I do like simplicity so that I can focus more on role playing.
You will find the best explanation of the skills on pages 174 through 178.  They are listed and described under the relevant ability score first.  So if you want to look up the skill for climb or jump, you would look under the section titled STRENGTH.  This doesn't mean that strength is always the appropriate ability score modifier to apply to jump or climb checks. Based on the situation your DM may use, or allow the character to use, dexterity instead. 
I hope someday to see a list of the actions my character might want to perform such as climb or swim and the suggested ability score to go with it.

From D&D 3.5 to 5.0(5E) - 11 - Traits, Ideals, Bonds, and Flaws

I mentioned traits, ideals, bonds, and flaws in my last post with little detail.  These four attributes are all part of your character background.  I'm sure your DM would allow you to skip choosing one, but I think they are a great aid for those of us desiring to role play our characters; and by role play I mean that we want to get into the mind of our characters and try to think like they would think and act like they would act; not simply do what I as a player knows is most likely to lead to success and rewards.

Here are a few examples of each from the sage background.  If you choose the sage as a background for your character you have:
Eight traits to choose from, two of which are:

• I love a good mystery
• I am awkward in social situations

Six ideals to choose from (ideals also have alignments they best apply to), two of which are:

• Knowledge: I can improve through knowledge (any alignment)
• Power: Knowledge is the path to power (appropriate for evil alignments)

Six bonds to choose from, two of which are:

• I've been searching my whole life to the answer to one question
• It is my duty to protect my students

Six flaws to choose from, two of which are:

• I am easily distracted by the promise of information
• I overlook obvious solutions in favor of complicated ones

From D&D 3.5 to 5.0(5E) - 10 - Character Backgrounds - A BIG change!

Character backgrounds may be the biggest change in the process of character building for those of us moving from 3.5 to 5.0, and it is an aspect I believe will be very helpful.  The 5th edition places more emphasis on pure role playing and less on game mechanics and character backgrounds assist with that.  You can create your own character background or use one of the character background templates provided. 

Character backgrounds don't just give your character a back story, they provide your characters attributes that measurably affect the mechanics of the game.

• All backgrounds provide two skill proficiencies.
• Almost all backgrounds grant the character proficiency with one or more tools.
• Many backgrounds grant the character an additional language.
• Many backgrounds provide your character with equipment in their possession.
• Most backgrounds grant features to your character related to their background.

One background provided on page 139 of the PH is the Sailor.  Players that decide their character should have the sailor background will see that their character gains proficiency in the Athletics skill and in the Perception skill.  The character will also be proficient with navigator's tools and with operating boats.  They will have some specific equipment such as a 50' silk rope, and the feature they gain is the ability to find free passage on a ship for themselves and their companions.

All background templates also include a list of suggested Personality Traits, Ideals, Bonds, and Flaws for your character to choose from.  As with all things D&D, you can work with your DM if you don't want to follow the suggestions provided in the template.