Were I to run 4e I'd go with the 9-alignment system and do as I've pretty much always done: let players run whatever they want. And if one player brings in a Paladin and another shows up with a Necromancer there might be a PC down before the party even takes to the field! (and I'd quietly be cheering for the Necro.; Paladins - as pointed out upthread - when played welll can be more annoying than any other class)
I don't care if they kill each other. Hell, it makes my job as DM easier if they provide their own opposition for the session and I don't have to do it!
We had a run a few weeks ago where an entire session and part of the next were spent:
1. tracking down and killing one PC who had fled from combat, stolen some of the party's stored treasure, and taken off;
2. introducing another PC and his companion NPC to the party;
3. killing off the newly-introduced PC on discovery of both its race (roughly equivalent to 4e's Tiefling) and class (Assassin);
3a. this turned into an all-out brawl involving all 9 party members;
4. killing off a third also-new PC that had tried to defend the second one.
And great fun was had by all; including but by no means limited to the player to whom *all three!* slain PCs belonged! (same player came right back with two Lawful-Good types; he doesn't always play nasties) But yes, a Paladin trying to join this party would either die or run screaming for the hills...
In case it's not already obvious, characters trying to enter parties in my games don't come with little "PC" stamps on their foreheads...
Lanefan