STO from Cryptic is the only MMO I've continued to play after beating the main storyline/maxed my level. I've tried TOR and Rift, and honestly had no interest in continuing either after hitting 50. The former, I made several other characters to level 50 (they each have voice-acted, scripted storylines, probably why it cost so freaking much), but after completing those, I havent logged on in months.
I honestly didnt give a damn about what little story there was in Rift. Beautiful Artwork though.
The storyline for STO starts out interesting, but near the end "Undine Advance", it really gets shoddy story-wise. The main appeal it has is the AWESOME space combat, which TOR's lame space missions cannot come close to matching, and ground-based combat like Rift bores me.
STO certainly isnt perfect---1. ground missions/battles are awful mechanically, but like I said, the typical MMO ground mission in general doesnt appeal to me.
2. The graphics can be awful at times, even running in the beta DX11 mode.
3. Servers can be a bit iffy at times