I don't know, but the original R6 and Rogue Spear really did it for me. They were just fun, you know? SWAT 3 was a great title as well with a different angle to play. Since then, while the R6 games have added more features I used to want (like viewable weapons), they just feel worse to me. I don't know. Maybe I'm nostalgic but I sort of liked the streamlined interfaces, easy to use multiplayer, smooth/fast action that was unforgiving and violent and coop that made your eyes bleed.
Does anyone remember the Defend coop gametype from one of RS's addons? (I think it was the Urban Ops addon). Each regular map in this gametype had a central location selected with a big red button added into it. You spawned in around it and the AI basically had a "get to the button and hit it at all costs" mentality. They would assault from all sides, toss grenades... Great stuff, and the only way to win would be to prevent them from breaching the room and killing them all off.
How is it that since then we get more patches, less gametypes, slower and more annoying interfaces and braindead stuff like Black Arrow? Hell, R6 was faster-paced than Black Arrow without sacrificing gameplay options.