I play a mission in STO about once a week, but I check in every couple of days for DOFF assignments. I don't have the time to make any subscription game a worthwhile investment, it is just wasted money for me. STO is pretty solo-able, F2P and I'm a bit of a Trek fan, so it is a good fit for me. Also the space combat is tolerable. I've dabbled with LOTRO F2P but at the time I was a bit fantasied out.
If you want to look outside the MMORPG genre and just consider MMO games, Planetside was awesome. I have every hope that Planetside 2 will be just as awesome. Fingers crossed for an Oceania server.