For one, you only play with 6 players on a team in a game with 9 characters. Teams are almost always 2 soldier, 2 scout, 1 demo, 1 medic. Yeah, I know most other characters are terrible at a high level (much like tiers in SSB) but why not play Highlander mode? At least it'd be better than the borefest of Scouts, Soldiers and Demos all the time.
Two, you only play a a handful of maps (but I guess that can be said with SSB, too)
Three, TF2 is slow as hell. It's basically the Brawl of FPS games.
Every class has their role and if the team synergies well it can beat many standard teams. The only class that I can say is bottom tier is the fat *** heavy that often drags people down with him because he is such a slow fat ***. He is decent on defense though.
For example, Spies can turn the tides like crazy and work well with teams that distract the enemy using scouts.
Pyros have great utility (but not in all maps) and are great ambushers, reflectors of projectiles, and like spies are great if say the scouts distract the enemy
Snipers can single handledly clutch the game and are very viable on 90% of maps, many pro teams start off games using double sniper on certain maps to **** the heads of people as they rush to middle. Snipers can force a medic and his buddy off a point where a spy or scout can sneak in to ninja the final point.
Basically, it all comes down to good teamwork. I will admit though that most top teams follow the standard formula with some European top teams having whack lineups like a constant heavy.
Highlander mode, more people should play yes
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Handful of maps isn't bad, a small number of good maps is better than a large number of crappy **** sucking ones (for competitive play).
People are definitely trying to solve this problem by making more cp maps that are experimented on people in random leagues, some of the maps are actually pretty good like Cp_follower, others are bad like cp_science
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TF2 isn't slow and if you think about it, it's more fast paced then CS 1.6, one of the most popular, skill-based fpses, where meticulous strategy, often slow-paced movement usually happens before the action occurs.
Almost all maps in TF2 is a crazy rush to mid in the beginning where demomen, soldiers blow themselves up to get to middle and (often) the 2 scouts battle the other 2 scouts in the middle. So many different variables can happen in every single middle fight as well as the rest of the round.
So it's not really like the FPS of brawl either, to me it's an actual decent FPS spectator sport where other FPSes are unwatchable.
Now CS: Source or DOD: Source are more like the brawl versions of the original CS 1.6 and DOD.