I hate to compare PSN to WoW however, i know a lot of people including me very much enjoyed the completely pointless title rewards for completing specific achievements.
Just little single words or phrases which appear before your name so say:
Kill 10000 TR
"TR Slayer" kamikava
Or successfully defend a base 200 times
kamikava "The Defender"
etc. you know what i mean.
Just think they're cool haha
All of these carrots need to be ultimately pointless as any buffs will give unfair advantage to long term players, i think it is paramount that this game remains focused on a players skill not what gear you get for completing achi's. Carrot are just something else that will keep people interested.