Q3A and UT99 were great fun. The problem was: the developers of those 2 games had no clue whatsoever what made the game fun ! So their followups failed some of the basic gameplay design issues.
You might think I'm crazy. But I really think that was the problem. Look at Tim Sweeney and John Carmack. Those 2 guys were brilliant. But they were only interested in rendering and engine technology. They hardly cared for the games themselves. I bet most of the level designers were more busy with the architectural esthetics of their maps, or the technical details and limitations, than that they cared about gameplay.
I played a lot of UT99 CTF, back in 2000. Great fun. The greatest achievement of UT99 CTF was that both noobs and pros could play together on the same public CTF servers. The pros would of course decide the games. They would be the ones grabbing the flag and capturing it. Or making that last second recapture. But the noobs would be able to do something useful. Even if they had to stay in their own base, and play defense only. They would have fun, and would even be able to make themselves useful.
The reason was that UT99 weapons were spammy. And dodging gave you a big advantage over noobs. But it wouldn't make you invincible. A very good player could go into the enemy base, kill each defender twice, grab the enemy flag, kill 3 more defenders, fight his way out of the base, and then still get killed by a lucky 8-ball combo from a noob. Or walk into blobs of goo from the goo gun, which was spammed there 60 seconds earlier. The good players had the advantage, and win 4 out of 5 fights. Or 9 out of 10. But the new players did have a chance, and did kill the better players. Enough to be slightly effective. And surely enough to keep the game fun and interesting for them.
With UT2003, 2004 and UT3, all the weapons changed. Spammy weapons were not powerful anymore. Hitscan weapons were the only real weapons to use. And dodging became so powerful, you could never hit a good player with a asmd-combo or 6 rockets from an 8-ball. The new players stood no chance. Not only would they not have any way to make themselves useful, they also would end matches with zero kills. Not fun from them. And they would stop playing quickly.
I don't think Epic Games (or id) realize this. They listened to the top DM players when developing their new games. And those top players want no spam, and very powerful and fast movement. Because they hate it when a worse player kills them. They want to avoid that at all costs. But they forget to realize that the other player also need to have fun.
So what needs to be done, to make a successful arena game ?
1) Make a game that both experienced and new players can enjoy. Even when playing together in the same game on the same server. This can be done via weapons-design. And via map-design.
2) Public games must try to balance both teams. This can be done via an elo-like system. Give every player a permanent id. Maybe allow them 2 or 3 different nicknames (so they can play anonymous if they want). But keep track of their performance/rating. Use those ratings to balance teams. Use those ratings to let players of the same skill-level play together on public servers. Allow good players to play with their noob friends if they want, and use the proper math to reflect that in the ratings. Give high-rated players a title others can see. That would be incentive/reward for the good players to play.
3) Release new maps on a regular basis. One of the reasons that arena shooters grow stale, is because you keep playing the same maps over and over. Custom player-made maps do help a bit. But if a company would release 1 or 2 quality company-endorsed maps every month, and arena-shooter can stay fun for a year or more.
I don't think arena shooters are not fun anymore. I just think that the developers don't really realize why they were fun.
(Note: my apologies for using the word "noob". In don't know a better word in this context. I mean a new player, or a player who isn't very good (yet)).