All of the Unreal Tournaments have in common the basic mechanics and the trigger-happy, reflex testing gameplay that fuels your adrenaline.
But the first one has the distinction of being the one that got it right - the balance between an addictive gameplay and the perfect threshold of complications, game concepts which steepen the learning curve. There are just enough game modes, weapons, maps, customisations, modifiers, etc. to provide a varied experience without overwhelming newcomers.
It's easy to learn and much harder to master, but it's a fun and balanced experience for all skillsets. The graphics are vastly customisable and will run on any machine while looking good. The flashy graphics of the day have downscaled to sober and functional graphics today which still look very pretty.
The adaptive AI is impressive when you're offline but this is best experienced with some friends in the same room - get them to buy it off GOG.com too and you'll wonder why you ever needed DirectX10 - or any other game for that matter.