Great concept gets lost easily in boring, overly complex team based game play.
There are different type of maps with various objectives, for example, capture the "intelligence" or better known as capture the flag, pits team red vs team blu against each other. If you're a seasoned online FPS gamer, this game mode plays exactly like any CTF you've played and really has nothing special to it.
Another popular game mode has one team on the offense and the other defends wile the offense team pushes a cart to a checkpoint to win. I found nothing exciting with this concept and it gets boring very quickly as the rounds don't last long.
To the subject of maps now. The play area is usually very tight, I had a claustrophobic feeling most of the time when about half a dozen of my team mates all crunch up together and we lose track of who's who and what's happening, this happens frequently and really is no fun at all.
The character's abilities feel very useless on their own but are increased with modifiers from your team, for example a medic will heal a machine gunner while other team mates rush in for a kill. The engineer can build support items for the team to replenish ammo and health, and also defend map areas with a sentry gun. Those are just a few examples, the game mechanics function to encourage team play and discourage rambo-one man-army types. This is the aspect of TF2 that makes it what it is and can be easily broken if a couple of people on the team are not doing well, causing the team to lose a game fairly quickly.
The graphics and sounds are nicely done, the characters look aesthetically pleasing and are fun to play with, the textures are very soft and easy on the eyes. Sounds from weapons and speech are also fun to hear, the ambient in some parts is well made.