The game's network code is quite solid (a staple point for the Unreal engine since Unreal Tournament was released), but I encountered a number of quirks involving general jumpiness as well as some weirdness involving vehicles (turrets facing the wrong way) that will need to be smoothed out. The graphics engine itself was probably the best asset however, as even at maximum detail (some graphical settings are limited to a "normal" level to cut down on size for the demo) the game totally screamed on my XP2500+ Radeon 9600 Pro rig and failed to crash no matter what I did to it. I think the only significant technical problem I had involved the game's audio - the music and sound effects had a very hushed feel to them that made simple noises almost impossible to make out, even after I jacked up the volume. This is a common issue that many people have, one which will hopefully have a fix in the very near future.
There's so much more I could go over, but with the brief time I have played Unreal Tournament 2004, I don't feel qualified to cover these aspects in great detail yet. Suffice to say, the game is shaping up excellently and this demo astonished me by exceeding all of my expectations. What I was prepared for was a slightly updated version of Unreal Tournament 2003 that I would brush off in an instant, but instead I got something different and refreshing enough that I find myself quivering with anticipation for the final version. The full version of the game promises a huge number of new features, the usual vast support for the third party modding community, over one hundred maps (!), and will ship on either 6 CDs or a "special edition" twin-DVD set (and reportedly has a 5GB install footprint - ow). Start clearing up that space - the game is expected to hit shelves this coming March.