I prefer my RPG's single player, because in the multiplayer ones Ive found that there is an absolute deluge of morons who have one goal, and one goal only, to spoil everyone elses fun.
Player killing for no reason comes to mind as the major annoyance, I dont mind PK if its part of some plot or roleplay but its rarely that, its just some greedy 11 year old punk with a well tooled character who thinks he is "leet".
With single player I have control, I can play how I want and dont have to worry about some selfish moron spoiling it for me. I dont mind adventuring in BG, BG2 or NWN with a small party of close friends because I know them and trust them to roleplay. Apart from that though? Id prefer tightly plotted single player game anyday. Anyway, back to the topic.