Anyway, my take on the subject:
Someone told me there are two kinds of reasonable DM's: Those that start with core-only, go from there and loosen the thumb screws individually. The other who start with anything allowed and start tightening the thumb screws individualy.
In his opinion people who did otherwise (eg:setting a bunch of specific sources and never deviating from it) just weren't fit to DM.
If this statement were true, I'd find myself among the second type. I name a campaign setting and say: anything allowed, but better double check as I don't appreciate punpun running around. So it's really the obvious individual things that get banned with me. I'm not going to let the punpun-worthy tricks loose in my campaigns. I always check if things are compatibel with the setting, as j0lt says, "no paladins in dark sun", etc. But even then, if specific campaign, area or god related classes or races want to be played, I don't mind reflavouring: Radiant Servants or Ruby Knight Vindicators mayhaps, why not?
And when someone asks to play something which I don't have acces to, I make a minimum effort to find that source, if not I tell them I don't have any way of knowing what it is and they should make sure I can see the source before they play it.
The last is a reason with which other DM's might bother me alot. I think that "I ban it because I don't have the source" is a good reason to ban, but ONLY if you make minimum effort to find that source or actually read it when the player gives it to you. I have know this guy who keeps declining my offer to read almost every resource possible due to frantic lazyness (I'm not saying he should read the lot, I'm saying that if you think you're a worthy DM, you might eventually get to learn more sources except the ones you thought were cool and used on your own character in a previous game), who then goes banning everything because I do not have any proof it exists.