One thing that surprised me (a bit off topic I guess) is that the classes gain levels at different xp. Obviously this is an OSR thing, and this and shadowdark are the only OSR for things I've done (except for living through the original old stuff). Anyway, I'm not sure that dynamic is something I want to relive. I was curious what others thought about that.
Yeah, it goes back to TSR D&D. You can see it today in a variety of OSR games, including Castles & Crusades, OSE, OSRIC, Swords & Wizardry and Basic Fantasy, most of which explicitly emulate TSR D&D.
The theory is that player characters are balanced by total XP, not level. So a thief might be higher level than a magic-user, but they have comparable XP and they are, in theory, comparable characters.
This is probably an optimistic take by the designers in most cases and I think it's easier to balance the classes -- if that's something the designer wants -- to do so by level instead. And once you've done that, there's no reason to have varying XP tables.
I've done it both ways and as someone who does want my players' characters to be roughly comparable to one another, and who has seen how it feels for some players to have their characters be behind in level, balancing by level, with a unified XP chart, is the way I prefer.