3e charge. It’s probably the common rule my players “forget” is not in 5e
Yeah I just looked this up the other night while playing AL. I was rather surprised it wasn't in the game, especially when there are monsters (I'm looking at you Skeletal Minotaur) that have "charge" as a special ability. I mean, there are reasons you don't need charge anymore, you can now move and attack all you want. Sure, you can't double your movement unless you're a Rogue or Monk with Dash as a bonus action. The lack of Charge, but the ability to move freely and make
all your attacks is a bit of a tossup for fighter-types.
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I miss 3.5 templates. They were terribly balanced and they're not difficult to create, but I really liked being able to grab a template and slap it on a PC or NPC to make them more interesting. There are some limited examples in 5E (were-creatures), but they just feel lacking.
Also: 5E monster design. I feel like 5E monsters are this weird middle ground between 4E and 3.X that just...doesn't satisfy either. They either seem bland and generic and needing of feats, classes and "stuff" (dragons in particular) or they seem like they're trying to build a monster that mimics a class, but without the class features (most of the humanoid enemies). Why not just make humanoid enemies have classes? And "special" creatures have "special" abilities? I don't like whatever middle ground 5E is walking here.
Also also: Delay. This should be just a standard rule in RPGs everywhere. If you don't want to take your turn yet, or aren't ready to take your turn, or whatever other reason, you should explicitly have that option.
Forcing people to act on their "turn" doesn't really strike me as something the DM really needs to be burdened with enforcing.
Also also also: An Index that MAKES BLOODY SENSE! I hate the 5E index! It's horrifying to try to look anything up. It really doesn't help that related rules are often scattered around the book.