I take it you don't play UT then.
I take it you never played Tribes?
Here's why Tribes requires the most skill:
1. Basic movement involves far more than simply strafing and hopping around. You have vertical movement in the jetpack which takes a lot of practice to master. You have to learn how to ski, which consists of you gaining momentum while hopping on declines and then jetting out of inclines. To do this, you have to learn the intricacies of the terrain, memorize cap routes, etc. Momentum is key in Tribes and once you get going, there's no quick way to stop (other than hitting a wall).
And then of course you have the various "trick" things you can do to gain momentum, like disc-jumping or grenade jumping in midair. Defensively, you have to learn how to bodyblock, which is basically predicting an incoming enemy's trajectory, then getting in their way and stopping them dead in the air.
2. Aiming requires far more leading and prediction than any other FPS. Most shots are made at long range and, combined with the slow firing rate and projectile speed of most weapons, requires you to compensate for the velocity of both you and your target, the angle of the shot, the distance between you and the target, etc.
You've also got mine-discing, where you throw a mine at an enemy, then shoot the mine, causing massive damage. Keep in mind that most of these shots need to be done in midair because that's where your opponent will be most of the time.
3. Teamwork is far more involved than any other FPS. You have to not only defend your flag but also maintain your base and attack your enemy's. Your base has numerous assets, most of which are invaluable and interconnected. For example, without any functional inventory stations, you can't select a loadout, which means your cappers won't have energy packs, you won't have any snipers defending the flag, no heavies sitting on the flag or spamming the enemy base with mortars, etc.
Cappers, Heavy Offense, Light Defense, Turret Monkeys... all emergent roles created by players throughout the game's evolution. You can't beat another team without people playing all these roles. A capper will never be able to grab a well-defended flag without support from HO, for example. Even if they grab the flag, they'll still need support from fellow cappers in case they get sniped or chaingunned down. This is where synchronized midair flag passing comes in. Two or more cappers have to time their capping approaches perfectly to pull this off, as well as time the initial grab perfectly so they don't get killed by friendly mortar spam. Once the first capper grabs the flag, he tosses into midair, at which point the other capper, approaching from a totally different angle, grabs it, throwing all chasers off guard and off balance.
So yeah. That's just the tip of the iceberg. Some of the stuff may sound easy to pull off, but they aren't. Play the game and see for yourself.
Or better yet, watch the three parts of the Tribes: Legacy video and try to replicate those acts yourself.
http://www.youtube.com/results?search_query=tribes+legacy&search=Search