Lickers and Eliminators. They're small, agile, fast, and fond of leaping attacks.
Hunters too: big, agile, fast, and fond of leaping attacks.
Ensemble Darkhorse: Fan favorite HUNK is back and gets his own chapter in the game which is like an updated version of The 4th Survivor. He also gets some very memorable lines in the game as well.
Richard Aiken was one from the original game is featured in a chapter as a NPC partner for one of Rebecca Chambers' segments and receives a little bit of a tragic background reasoning for joining S.T.A.R.S.
Rebecca and Bill Coen both get a go in the Resident Evil 0 scenario and the fans to this day desperately await the day they will main another title in the main storyline of the series.
Goddamned Bats: The leeches in Train Derailment, small spiders in Mansion Incident and the roaches in Raccoon's Destruction have the potential to be like this. However, only a small number of them actually attack, in which they cause no damage unless you leave them on, and can be removed with a simple swipe of the knife. It's mainly if you're overwhelmed by other enemies at the same time that they become annoying. Played straight with the crows.
Scrappy Mechanic: The player must replay certain levels and score an A rank in order to unlock the final level, adding an unnecessary layer of padding in the eyes of many. Thankfully removed in The Darkside Chronicles.
The Ivan boss has the potential to be a real pain, and it gets worse in Dark Legacy where you have to fight two of them simultaneously.
T.A.L.O.S., particularly with its second form's abundance of Press X To Not Die attacks, has major irritation potential.
That One Level: Umbrella's End Chapter 2. The once-per-level life spray is not in an immediately obvious location, and herbs are not positioned very intelligently (showered in them when you don't need them, nothing but ammo and more goddamned Hunters when you do).