I have three things to add, first two on topic, third one not...
First one:
FPS games on the whole have to be very careful about not giving away the entire cake with the sample.
So I imagine an open beta will contain SOME of multiplayer maps, and none of the single player missions (or maybe one). It will be like the RTCW multiplayer test. It gives a whole lot of work towards increasing game balance, and a sample of gameplay, without giving away the entire game for free. Multiplayer tests are my favorite form of open beta.
Second one:
Much of the game balance and obvious balance flaws should be hacked out before it ever hits beta. The less gameplay changes needed, and the shorter the beta period required, the better off you'll be. Nothing kills someone's fun quite as quickly as major nerf to their favorite weapon, two days before the game goes gold.
Third one:
UT2K3 does not suck. In fact, it's eclipsing UT now. Damned fine game. It is sad they changed the pace of the game (the weapons are noticeably weaker, and therefore frags aren't quite as shockingly rapid), but on the whole, they added the double jump and wall jump, which more than made up for any mistakes some people may believe UT2K3 has.
So for UT2K3, I'll chalk it up to a slightly hasty release (for the first two patches, the netcode was shaky), and a general light nerfing of the pace (same thing happened to T2, the pace dropped.)
But when people found the new and hidden depth (oh man jumps in UT2K3 are fun), you learn exactly how cool it can be.
That said, T2 didn't have any hidden depth, exception being the shocklance, which was also an overpowered piece of crap that unbalanced the gameplay.