I played a bit of search and destroy, conquest, and CTF when I bought warband ... Basically, search and destroy is just like siege, just harder for the defending team.
Conquest and CTF are both pretty cool, but the only native map suited for those gamemodes is ruined fort (and perhaps field by the river). So as chadz said, we'd need some more maps specifically designed for those.
Conquest mode could be really awesome with a battlefield-like map.