Verbs in fight scenes are really crucial, since that's basically how you're going to get all the action across. If you use the same verbs over and over again, like "punch" and "kick," it gets really dull really fast. Of course, you have to use those words a few times, but after one or two uses the reader will get that the characters are punching and kicking each other. I try to use words like "smash," "connect," "blast," and "puncture" to keep things interesting.
You have to know exactly where in time and space these characters are fighting. What's around them? Is there anything lying around that someone could use as a weapon? How are they going to move around the setting during the fight? Is one character a lot stronger than the other, pinning his opponent down and pummelling him, never allowing him the chance to get a punch in? Or is the fight more evenly matched, more dynamic, with more back-and-forth?
3. Emotional Impact
This is by far the most important. Why are they fighting? Why is a physical fight the best way for these characters to interact right now? If these characters don't have a good enough reason to be fighting, we're not going to care who wins. If the issue could be easily solved with a negotiation, why are they beating the crap out of each other? It has to make sense. When two characters come head-to-head for a real reason, when there are real stakes and the emotions climax, that's when your reader is glued to the page. If the emotions between the characters have been slowing building, getting hotter and hotter, and they finally come to a head and everyone breaks out into the most animalistic kind of fighting, that's when the fight works.
Think of fight scenes as love scenes. Is there a connection between the characters? Is there a lot of build-up? Are the emotions coming to head in a way that makes sense? Is the choreography easy to follow? How graphic do you want it to be? Wow, fight scenes and love scenes have a lot in common! ;)
Do you like writing fights? Or do your characters solve their problems in more civilized ways?