i agree with the thing about speed in solos. it's a really big crowd pleaser to be fast and amazing, but it's also great to be slow enough to make some sense. another thing is that if you play at slow or medium speed, it will be easier for your fans to play your stuff without skill. i know that someone is going to think that i only put that here because i myself am slow, but i'm not :). a good example of that would be joe satriani. he does slow and fast stuff in his songs. basically, i'm saying that instrumental stuff should be fast enough to drive people nuts, but slow and beautiful enough to move them to tears.
i'm a real romanticist, aren't i ;)