I really enjoy CS:GO.
Replayability: Totes. Online matches always fun, plus lot's of modded servers like found on www.joinsg.net!
Progression: Yeah, I mean the more you play the better you get. PLUS some modded servers, like you can find at www.joinsg.net, have credit systems where the more you play you get credits which can be spent to buy cool hings in the in-game shop!
Room for improvement and skill development (Competitive ladder or Unlimited Progression): Guess I kind of answered this but yes, CSGO has a skill devlopement that's pretty good. You can even try to be ranked in skilled matches. I would suggest trying to play with some good friends to get better. I normally find some cool people over at www.joinsg.net!
Community: Tons. So many it's hard for me to name them all--so I wont. Except for www.joinsg.net.
Developer care (updates or expansions): Always updating the game. Even though it's a bit old they just released a new version for a classic map. Plus communicates have "mappers" who actually build new maps to use in game. I know of a few that are really good, like Thorgot, over at www.joinsg.net!
(if it runs on a low-end PC would also be a big + but not necessary): Hell yeah it goes. Also, www.joinsg.net.