Thanks for a the great responses folks! As for me:
If it is a serious one, I like games that are unpredictable.
I like patrolling for (and catching) sneaks.
I like it when someone breaks things up and does something that makes me think 'Wow, I should have seen that coming!"
I like chasing down droppers and repelling campers. Heck, I even like getting riled up by a spawnkiller and making him rage quit!
It's nice to have a good kdr but I prefer a real challenging game. Uneven can be fun and I prefer to be on the low side but it can also be amazing to watch a great defense run by a few outnumbered reds, fighting tooth and nail with their back against the wall. (I miss Blue Max)
Banter is great when it's respectful. (Complaining and provoking ruins things more than 'poor sportsmanship'.)
I also love casual games. Circus sessions and goofing off have been some of my best memories here. Dodging noobs in a dogfight without shooting back can be a blast! I wish I could get points for 'making' a persuer crash into the bridge.
Exploring the underground, trying to land in impossible places (like on a zeppelin or on top of a building) having ceasefires and sharing tricks....
These are a few of my favorite things.