Do you typically DM and/or play in kitchen sink settings, or limited settings? That is, is your group’s setting one where any PC race and class is pre-approved or are certain options just not available? Which do you prefer?*
I am our group's primary dm; I run a limited setting, but one that is fairly open.
Best example: There are no Drow pcs in my campaign. Once in a while I'll have someone play an npc Drow or something, but Drow are monsters, not pcs.
If you like limited settings, feel free to answer any or all of these follow-ups:
Does it matter whether the restrictions stem from DM fiat (“I just don’t like sorcerers”), from setting themes (“The gods have abandoned Athas, so no divine PCs”), or from print origin (“Nothing from the Essentials books”)?
Not in practical terms; they are there, and that's that. (As a player I feel the same; it is the dm's game.)
Personally, I line-item ban for balance or setting reasons (Drow, for instance, are a highly mysterious, almost-never-seen race in my game, similar to how they were in their first appearance as monsters in G3).
I also have a philosophy of trying to let players do what they want, so many things I restrict are under what you might call a 'soft restriction,' where the first player who wants to play one gets to and then it's banned due to rarity. (The shardmind from the 4e PH3 is a great example; the racial flavor doesn't match my campaign setting, so I rewrote them as ancient constructs from an old empire known to use weird crystal technology; any surviving shardminds are either ancient, or were 'off' or in stasis for tens or hundreds of thousands of years. Thus, they are very rare, and I'm not letting anyone else other than the guy currently playing one play one for quite a while unless they somehow come into a place or time where it makes sense for there to be another one. A lot of the (IMHO) extraneous races from 3e fit in this category for me.)
If you like strongly thematic settings, is such a setting any less acceptable if your favorite races or classes aren’t part of the theme?
If there's nothing I'm interested in playing, I'm not interested in playing. That said, I'm not generally too hard to please on this score.
What about strongly thematic settings that naturally foster a kitchen sink attitude, like Planescape?
Nothing wrong with them, or with kitchen sink settings in general, and I'm happy to play in them. (Over the years, I've integrated more and more Planescape with my regular campaign, but still at mid and higher levels; I'm not a huge fan of 1st level planar stuff.