Graphics engine
Spectacular use of a flight sim engine for a first person shooter set in a distinctive location
Settings
A surprising amount of variety considering it's all set in corner of the world: from rivers to cities to bunkers to hills to open desert
Multiplayer
Once you get past the Novaworld interface, solid netcode and varied gameplay modes give Black Hawk Down serious multiplayer longevity.
Single player
Some great scripted moments and spectacular firefights, using limited saves and lethal weapons to heighten the tension
Can I ride with you guys?
Cue the Ride of Valkyries
Thanks for flying Air Army
Cons
Single player
Since it's ultimately limited and awfully short, it's going to be disappointing for people who aren't interested in multiplayer (if, in fact, there are any such people).
AI
Your teammates are almost as dumb as the enemy militia, who apparently learned their tactics from Galaga
Weapon physics
Sure, they look and sound great, but these guns are far too easy to use in a Quake-ish run and gun manner