This is a good point. I mix in key and the transitions are much more seamless (especially for mashups - you'll be surprised how many people don't think about this). I had this conversation with someone a while back and he said that mixing in key limits your choice in what music to play. Dumbest thing I've ever heard. I can still play any song I want to. I'll work around it until the key's work together to make a transition, rather than playing the track straight away.
As per the other posts, most people who DJ imagine themselves playing in the big leagues. Nightclubs aside, a wedding / private function DJ needs to have the same qualities as the ones I've stated in my first post. IMO they need to hold more respect compared to a club DJ, as the clients and audience are more intimate. The only outcome I could see where most of those points wouldn't apply is playing radio. DJing should be fun, which is the reason most of us get into it to start with, however, if you're looking to take it seriously you're gonna need to be prepared