In this thread I will bring together all aspects of the knowledge of what game modes will be present, how the game will feel, mutators etc as described from hands-on demos supplied to games websites and magazine writers.
What's In:
Deathmatch (presumably Team DM too!)
Capture The Flag
Onslaught
Assault
Conquest (Apparently a mix of Assault and Onslaught where you control territory rather than just nodes)
Instagib Mutator (Of course

)
What's Not:
Double Domination (Hooray!

) - Epic realised that Double Domination was frustrating and not at all popular!
Epic have indicated that there will be scalable graphical settings, so computers not entirely capable of rendering the kind of jaw-dropping graphics seen in screenshots and trailers will have an easier time. Gameplay will be more like UT99. A lot of the game modes incorporate vehicles, so there will be a good mix of on foot and in vehicle annihilation going on! Each map will be designed to keep both type of combatant with a fighting chance of completing objectives, with clear advantage given to neither.
You can contribute to these reference threads in the provided "feedback" threads (for this one it will be "HUTP: UT3 Gameplay member contribution"). This is mainly to keep track of information sources and to keep this page as close to truth as is possible at this stage of develpment. Once your contribution is confirmed I'll add it to this post ASAP

thanks
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Sources:
Gamespot
IGN
Gamespy
Unreal Tournament III Offical Site