D&D 5E Hurdles from D20 to 5e?

[Croesus]

5E does not have the famous/infamous 5' step. This and no Delay action were the two changes my group had the most trouble getting used to, mainly because of how often they came up in 3E.

5E: If in melee range of an opponent and you want to leave their reach, you have to take the Disengage action, or they get an AoO as their reaction. Most creatures get only one reaction in a turn.

Spellcasters casting a spell while in melee range of an opponent have disadvantage on any attack roll with that spell. However, if the spell has no attack roll, being in melee range has no impact on the caster - well, no impact until the opponent smacks him upside the head with a greataxe.

Edit: one other thing. Most creatures have a lot more hit points than before. Don't send a single magic missile at each of three goblins and expect them to go down.

 

[Sword of Spirit]

One thing to keep in mind is that, depending on how your group played 3e, you might see a 5e feature and immediately think, "that's broken." No, it's probably not. Character abilities are just more powerful, less limited, and easier to pull off across the board than they were in 5e. Avoid knee-jerk reactions.

(One player who was brand new to 5e, but fully saturated in 3e, kept saying "that's broken" whenever something different like that would come up in 5e. He wasn't trying to be obnoxious, but he really didn't understand the game yet, and that was the result.)