Will is right, the groove is the thing. Fast is good – it can be very exciting – but if the groove's running away with you, it's probably best to take it down a bit.
The other thing is variety. One set of fast reels sounds amazing. Seven in a row, and they lose their impact somewhat. Same goes for slow airs or ballads. I'll happily listen to a ballad, but if it comes after six other ballads, well I'll not be looking forward to it so much. Mid paced minor key jigs? Lovely. Seven sets of mid paced minor key jigs in a row? – starting to get a bit tedious.