On my server(Sen'jin) almost everyone is overall pretty nice and the factions well at least horde is not too hostile towards alliance but I do not know about alliance towards horde.Some of the times there will be the occasional ass holes who go out of their way to get you killed. For example if a horde goes over to help an alliance who is getting his ass kicked by mobs and the alliance feels like being a total ass hole will most likely run and peel the aggro off him onto you so as you are trying to handle like 3 mobs the alliance runs away (grrrrrr), but the same thing could happen to an alliance trying to help a horde.  This is not the case all the time and some people will actually help you. I though prefer horde mainly because I think they are more lively.