Main new mechanic is advantage and disadvantage. Roll two dice and either take the higher or the lower. Does not stack and they cancel out all opposing instances.
Much lower money. Expect about 75 gold per level. This does not cover magic items but expect a lot less of them. Game is actually balanced for mundane weapons unlike 4E.
Attack bonuses are way lower. Level one you have +2 at level 30 it is +6. 4 points difference. Also note this means starting with 16 stat is quite powerful. But since all stats are useful it may be better to select many stats at 14 to maximize total bonuses.
Human with 13 in 5 stats starts with ten points of attribute. That is darn powerful. But unfocused. Focus gives you a way to get to +4 in one or two stats at the cost of being weak in areas outside your expertise.
You can break up your move however you like. Example and Archer can walk ten feet to take a shot move give feet to open a door then move fifteen more to cross the new room.
Getting up from prone takes half your move.
Moving only provokes when moving away
Criticals mean double number of dice. This includes sneak dice.
Allies and enemies both give cover. So you will want clear shots but movement makes that easier.
Inspiration is like action points. It can be given to ally. It grants advantage. You should get it from your three role playing traits.
0 to negative max hp is dying.
Death saves are now first to three. Three successes you stabilize three failures you die. Rural 10 or higher on a bare d20. Rolling one is two failures rolling 20 means you regain 1 hp immediately. Taking damage after death is an immediate failure on a check.
Stabilization only requires a DC 10 healing check. If no healing is used on a stabilized character they go back to 1 go after 1d4 hours of being stable.
A blow that drops an enemy can be pulled to knock out rather than kill.
Flanking does not exist generally. First session I gave advantage but that is a only a variant rule and i am not generally using those.
Encumbrance rules are variant rules. Generally so long as you have less than STR*15 weight we don't worry about it under base rules so long as you have the strength needed for the armor you have selected.