This was the reason i played Battlefield 1942 for 7 years and only spent $50.
This was the reason i played Quake 2 for 5 years and only spent $50.
Desert Combat mod for BF.Rocket Arena, Urban Terror (F*** Counterstrike's ripoff of Urban Terror BTW) for Q2. Good Times.
LM2 was my favorite of all time, although RA2 was a big time killer. Once RA3 came along I was hooked as well.
I did BF42 for years too, the original DC mod was awesome, up until v.35 when they started nerfing the apache. I've not enjoyed a modern battlefield series game since (although I do have all of them).
I would wager that if you put a mod into any modern BF game that would allow people choose between the normal 'have to go through the whole class system and play weapons as they are unlocked', versus another version where people can jump in and play any weapon or class they want, all without having to 'level up', people would abandon the whole 'ranking and leveling' bullshit instantly.