Having a fairly combat happy group of players I have run quite a few combats. The one thing my players and I have learned is that combat in DFRPG/FATE can be fairly quick and brutal. Sort of like real life. Let's be honest, if you started a gun fight in your living room with six armed people, it would be about four to six seconds before all that was left was the bleeding and crying. Less if you had people that knew how to shoot and keep their heads in a gun fight.
In that example, it presumes that everyone just opens up and no one starts diving for cover or anything else besides "Stand and Deliver!"
As my players have learned, many of them the hard way, is that this system really encourages one NOT to go the route of simply swinging/shooting each round. If you get hit by something serious, in a serious fight (read: not just some bar room brawl of fisticuffs) it is going to hurt. Badly. So, one will want to focus on defense first in most cases. If you have some supernatural toughness/recovery powers, you may already have this covered depending on the magnitude of your opponent. But, if not, then you need to do what normal people would do in a potentially deadly fight. Use maneuvers to do things like find cover or other aspects of the scene to aid your defense or otherwise bolster your defense. Or, if you are a wizard, throw up that block if you don't think you can take care of your opponent in that first evocation hit.
The other HUGE factor that I have found makes DFRPG so hard for old vet players of other systems is how much of a factor teamwork can be. Many times it can be much more efficient and decisive for a group of players to work together, using maneuvers, declarations and assessments to bolster a big attack than they are to all pile on with their own individual attacks. Like one would do in, say, D&D.
For instance, if you had 4 wizards taking on a powerful sorcerer. The sorcerer has thrown up a big shield using foci and gotten a +14 block (5 conviction, +2 focus, +2 specialization, +1 mental stress, drops the fate point to survive and rolls +2 over on his discipline check). If the wizards are not real heavies themselves, breaking a 14 shift block by themselves get's tough if not impossible. They could each stand there and throw everything they have at him and just wind up down some mental stress for the trouble. On the other hand, if the first three play for the home team, drop some free-tag maneuvers, then the final wizard might not just be able to break the block/shield but could also do some stresses to the bad guy.
Say, a 4 conviction, +1 focus, +1 specialization, +6 for free tags (+2 from each of the other wizards) and then tags his own High Concept for +2 takes him to 14. If he is positive on his discipline roll with extra successes, he's inflicting stresses!
Team work pays off big in this game.