I never really considered the performance issues, etc. the worst problems with the game - though the mouse input as per release was horrid and last I checked they still hadn't fixed some issues like ADS scaling.
The game mechanics and UI are just too fragmented and sometimes not intuitive and often not unified in approach either - what works for one thing might not work for another - making it a chore or just frustrating to play.
What do you mean by that? I thought it felt similar enough to DX:HR - which was the best game I played of its time.