Here is what I have noticed:
Cat 5 and 4/5 races tend to start very fast. It seems like there is always someone who thinks he or she can win the race from the starting line. After a few laps, the pace seems to drop and everyone calms down. Every attack is chased. The only breaks that survive are guys who should be 2s or 3s but haven't racked up the points yet. Team tactics rarely come in to play.
Primes are hugely important, but no one attacks after the prime because everyone goes so hard after the free pair of socks that they are spent.
The finish can be a huge disaster - everyone sprints for the win, regardless of field position or ability - and there often is a crash. If I'm not in the top 10, I tend to sit up and let the field fight it out.
Cat 4 races and 3/4 races tend to start a little slower and hold a higher pace throughout the race. Attacks are still chased down quickly, though breaks are more likely to survive. Team tactics are more pronounced. The finish tends to be a little smoother. The higher pace at the end of the race will spread out the field.
Primes seem less important, and it seems like the field will let a half dozen guys will go after them, unless its a large cash prime.