One of the things that I have never been quite so happy with in the Old School Renaissance is the extreme tendency toward simplicity in character types - after all, in a humanocentric OD&D game, you're basically a Fighting-Man, a Magic-User or a Cleric. That's fine for one off gaming, but as players want to do different things it can be a bit monotonous.
That's part of what I like about Holmes, which adds the Thief class. It can probably use a bit of polish, but without the Thief you pretty much have no options for the player who wants to be something like the Gray Mouser. I'm okay with the Holmes Thief at this point, really. The "character skill" aspect of this is avoided elegantly by noting that Holmes has a "Remove Traps" skill rather than the later "Find or Remove Traps" - the others are mechanical (Open Locks, Pick Pockets, Move Silently, Climb Sheer Surfaces, Hide in Shadows, and the already existing Hear Noise). This integrates tightly into the D&D I already prefer to run.
But the "basic 4" classes leave a real niche uncovered that I think should be filled. I think the Ranger type, somewhat modified from the original presented in The Strategic Review (it's a bit powerful despite its claim otherwise), fits it. The Thief is good at some dungeon tasks, and I'll probably add scroll use for extra Gray Mouser flavor, but a tracker / archer type is really what the game demands IMO. I've run into more issues with OD&D when players got frustrated trying to play a lighter-armored archer than anything else. The ranger fits with the loner hero type; Aragorn is the obvious inspiration, but I think you could find a lot of rangers in post-apocalyptic fiction (Hiero pretty closely fits, I think, despite the fact that he's technically a priest). And it has a clear place in the game, which I think is very important.
For me the watchword remains "don't repeat AD&D, or do AD&D lite." I think a lot of the restraint with regard to classes is based on this, but we shouldn't kid ourselves. It was relatively rare in golden age (pre-1979) play that people didn't bolt on any and every character type that they could get their hands on. I think it has more to do with forethought and planning; I think the Ranger is a good fit for the kind of D&D I want to do, so I'm tinkering with the class. It's one thing to go back to basics at first, so you can figure out what your style actually requires, but to enshrine this as a principle is mistaken.