I've reason to think that the ability to defend your whole fort with only a few champions, and literally no other military dwarves, is something that'll be corrected in a later release. They're overpowered at the moment.
The military system is changed in the next release. I'll just give a few details to give you an idea:
Weapons now do damaged based on physical attributes like mass, density, and edge. They inflict damage differently, so one thing we won't be seeing anymore (supposedly) is massive instantaneous dismemberment of any and all creatures hit by a sword or axe. This should make battles more interesting.
Armor behaves differently, acting more like a layer of protection that can be punctured or bypassed by different types of damage, with plate mail and chain mail acting differently.
Dwarves are trained in different ways, supervised by instructors.
Dwarves have only one champion, appointed by a noble. The rest of your legendary military will just be legendary military.
Squads are organized completely differently, with schedules that can be stored and changed. Dwarves' on duty / off duty behavior is changed dramatically.
There are many more changes. For details, you should look at what Toady has posted. The upshot is that combat should be more varied and interesting, and combined with the new creatures and underground the game will obviously change dramatically.
It should be out just about any day, people. The list of remaining things only has 2 things left on it before testing. I'm predicting we will see it before the end of the month!