For me, it's definitely the forums and the community that revolves around it. I love some of the people here, and I've had so many fun memories playing MCSG and learning new things and meeting new people. I enjoy browsing the forums and sharing knowledge that I've gathered when playing the game itself, and discovering parts of MCSG that I never knew before.
TeamSpeak is also a great way to communicate with people you meet, and to be introduced to new people. (Don't worry, if you meet anyone who creeps you out, you can leave at any time or ask for a staff member to assist you.) You can find people who you can play games with, and and maybe even become friends.
In-Game, there's a unique plugin. A large variety of maps with all kinds of gameplay, you can choose to play on all maps, or just ones you like - you can leave the game if a map you dislike is chosen. You can play with teammates, or you can fly solo, and when you're out of the game you can spectate the games and fly around the map. You can track your records and statistics on the leaderboards, and also in-game with /records or /stats.
The donor ranks are not unfair, but do give some fun perks to use. You can disguise your name and skin to avoid being recognized, get extra votes for maps that you like, and much more. One rank even gets pets in the hub!
Gosh, I feel like a walking MCSG Advertisement. Someone slap me xD