I think a good hip hop dj needs to have a good track selection, but this applies to all genres, as a brilliant techno dj would still be poor or average when playing rubbish tunes.

Scratching is definelty good to watch, but it needs to be skillfully interwoven into the set, as just watching people beat juggle all night won't really make the party kick off!

I'd say definetly in actually mixing, a hip hop dj isn't as technical as a techno dj. I've just started mixing a bit of uk hip hop, and because of my techno backgound I immediately wanted to get the tracks in a mix. But the outro in a hip hop record is really quite short, so you have to be skillful to get the beat match in for that short period. That's why most djs just cut or scratch the kick drum of the next tune in. So most hip hop djs don't really beat match to the extent of techno djs, afterall the records are only about half as long!

I think a good hip hop dj though will keep a sense of flow in the mix through his track selection and order. But also I think the people at the parties arent' listening for mixes, they just want to jump about when a tune they like is dropped in. Whereas at a techno party, the true technoheads definetly listen for the mix, and hear when it comes in, and jump about when the dj starts cutting it up.