Reputation - I've already grinded Emperor Shaohao to exalted and I loved the grind. Made around 170,000 timeless coins in the process.
Achievements - I honestly wish there was more of these and that things like Rolo's Riddle went more into depth because it wasn't much of a riddle. It basically gave you the area and the last part was painfully obvious. Zarhym's Body was a fun little break too but there needs to be more achievements.
Proving Grounds - I've been wishing and praising about a system like this ever since Cataclysm came out. The very fact that I saw a thread on the official forums about people having difficulty with Silver is mind boggling to me. Does it make me a bad person to require people to have gold in their specific role to come to my raids now?
Flex Raids - A bigger hit than I thought they would be. However, if you run them with a pug.. be warned.. headaches will ensue. If you run them with a guild or a coordinated group, then it's a pushover. I'm not a huge fan of the LFR loot system, but oh well, it's an extra gear up opportunity.
World PvP - The Bloody Coin farming is the most hilarious thing i've seen in this expansion so far. The rage that comes from General chat when people get killed is beyond hilarious, especially the people on PvP servers.