Here's a quick cut/paste from Reddit where a couple of posters (redkat85 and wayoverpaid) were summarizing 5E:
*Defenses don't increase with level. Plate armor means d10 physical protection. This is actually a good thing, because...
*Attack bonuses go up slooooooowly. If you want to improve your attack bonus, assign your skill point to it with a level-up.
*Although it still doesn't use the Vancian Spell Levels and Spells per Day system, the biggest change to magic is that you never have to prepare the same spell twice.
*Casters cast spells by taking metaphysical damage. This is an enormous pile of awesome.
*Multiclassing is nothing like 3rd edition.
*Difficulty replaces all the fiddly +1/-1/+2/-2 conditional bonuses that made rolls so annoying in past editions. They both mean take a bonus or penalty. That's it.
*Feats are sparser and chunkier. They're called perks, and you get one each level.
*Spells don't power up with the X per caster level mechanic. Helps keep the spell list smaller.
*No more reflex, fort, and will saves.
*Opportunity actions have been generalized into a more general action framework, and a number of spells and abilities are reactive. A defensive fighter, for example, can take his action to impose disadvantage on an incoming attack to him or an ally adjacent to him. Makes for a great tank.
*Grid-based rules are not by default. There are rules to engage and disengage in melee, but you don't have to play with five foot squares.
*Rituals! This is a bit of 4e that made it into the game. Um....no.
*Fewer skills, and more interesting bonuses to skills. Bardic knowledge is called Informed, and you burn hero points to use it.
