Monthly subs.

Cash-shop MMOs try to sap more money out of you, and overall they cheapen the experience. Even when it's just fluff stuff, I've yet to see a micro-transaction-based game that I actually like (and that includes LotRO, which I had originally subbed to on and off before it went f2p).

I can see why it may appeal to some, but it's just not something I'll ever be interested in.