Like many already said: No evil. No evil PCs. No evil spells.
Other than that, I try to be extremely accomodating to the players. I pretty much agree to most of what the players want for their characters, as long as it matches how they role play the said character. My latest reject to a player was when he wanted to get the Saint template from the Book of Exalted Deeds. Its not that I don't allow Saints, but his character was SO not a Saint. It hurt his feelings a bit (he really thought it would be cool... LA and all), but we found some other alternatives that he liked.
We use stuff from a lot of different sources. My current campaign uses:
DMG, PHB, MM, MMII, Fiend Folio, City Works, Seafarer's Handbook, Dungeon articles, Dragon articles, Book of Exalted Deeds, Book of Vile Darkness (for DM use mostly... we've converted some of the spells to non-evil ones), Necromantic Lore, Miniatures Handbook, a lot of the Forgotten Realms books (mostly player-owned), Complete Warrior, Complete Divine, Draconomicon, Traps & Treachery, Path of Shadow, Path of the Sword, Spells & Spellcraft, and D20 Modern. And I know I forgot some others.
My current game has:
Female human were-rat Rogue/Guild Master
Female human druid (this is her first time with D&D... keeping it simple)
Male yuan-ti fighter/dervish variant (uses a hand-crossbow)
Male drow ranger/fighter with the Vow of Poverty
Male drow wizard (working towards archmage)
Male bugbear (after reincarnation) fighter/warmage/havoc mage/eldritch knight
Instead of limiting the players to my ideas for a campaign world, I bring their ideas into my game world. It is challenging to bring it all together at times. As it turns out, my game world (this time) is somewhat like the Eberron setting. High magic. Not many "pre-set" alignments for races.
Who knows what I will do next time. I'm thinking about a city-based adventure with no armor and very limited choices of weapons. I like to throw in a point-buy system every now and then, just to make people think about their character concepts.