Sona offers two different support playstyles: being passive in lane or going aggressive for kills. The other supports mentioned can't really do that without losing their effectiveness.
Umm... I just played my first game as Thresh... I had to pay attention to NOT steal kills from my carry, and even then I did on a few occasions. Thresh can definitely be played aggressively. --- Now Playing: BL2, TF2, LoL i5-3570k | Corsair Vengeance 8GB 1600MHz RAM | EVGA GeForce GTX670 FTW | Asus P8Z77-V LK | Western Digital 7200RPM 1TB HDD