Most casters combine prepared and spontaneous casting /and/ at-will cantrips, including marginally effective attack cantrips. The exceptions are the Sorcerer, who's still just spontaneous, but gets some rationed free metamagic, and the Warlock, who gets at-wills and high-level dailies, but re-charges the bulk of his magical ability with a mere 1-hr rest instead of overnight.
Spell damage (and a few other things) scale with the 'slot' used to cast it (so you must cast a magic missile in a higher-than-first level 'slot' to throw more missles, or a fireball in a higher-than-third level slot to do more dice of damage). Conversely, Save DCs scale with character level, not spell level nor even caster level.
Instead of BAB, everyone gets 'proficiency' that goes from +2 to +6 over 20 levels. This applies to attacks, save DCs, /some/ saving throws, and trained skills. FORT, REF, and WILL are called CON, DEX, and WIS saves, respectively. There are also STR, INT, and CHA saves, but they are rare. Each class gets proficiency in one of the 'useful' saves, and one of the others. So you're going to have two important 'bad' saves at high level, unless you multiclass or use feats to shore them up.
Feats are /much/ 'bigger.' You only get a feat every 4 levels and must give up a stat bump to get it, but one feat is like a whole 3.x feat-/tree/.
Fighters get two more stat bumps than everyone else, the first at level 6, instead of bonus feats.
Fighters get old-school multiple attacks in a single standard action (simply called an action), instead of needing to make a full-round attack, and all attacks are full-BAB, not iterative - yes that makes them pretty high DPR even without much system mastery applied.
AoOs, and some other sorts of actions are consolidated into a single 'Reaction,' that you can do only once per round. A lot of cool things use Reactions, so you have to be careful not to have them get in the way of eachother.
Swift and other actions are consolidated into a 1/round 'bonus' action. For instance, you use your 'bonus' action to cast a spell that also lets you attack on the same round, or attack with an off-hand weapon, or a lot of other things, creating a bit of a bottleneck.
There are no wealth/level guidelines and magic items are strictly a DM-distributed resource. You can forget about tailoring items to 'builds.' Characters started above 1st level still just get 1st level gear.
MCing is slightly different: caster levels stack, to a degree.
Monsters aren't generally built using all the same rules as PCs, they have simpler stat blocks, more like earlier editions, though they /do/ have all six stats.
PrCs are gone.
You have a choice of 'background' (mostly what you did before you started adventuring) that determines some of your skills, some of your starting gear, and a perk of some kind.
