It's not you, it's me... I think...
Problem no 1: MMOs require a considerable time commitment in order to get to the higher echelons of the game. If you're not 100% happy with an MMO when you first buy it, it seems like a smart move to pull out early before you commit too much time to the grind.
This happened with me and Warhammer Online. There were some issues with the game I wasn't happy with, so I quit at the one month mark. The same happened with Aion. Issues, quit. Both games were not horribly bad, but because they used subscription models, I didn't feel like I should keep paying when I wasn't 100% happy. Why waste $90 US and 6 months of time, when you can be playing something else more enjoyable.
Some people like to refer to this type of behavior as MMO tourism. Well, I honestly don't like to tour anything, I just want to be certain that I'm getting a good source of entertainment for my time and money investment. You could think of it as an MMO shareholder really. If the entertainment value is potentially high, then I'm happy to invest.
Problem no 2: All the 'good' content in an MMO these days is at the endgame. When a new MMO is released, nobody knows what the end game is like. Should I take a risk and invest my time/money or should I move to something with a proven entertaining endgame (i.e an older MMO)?
To me it seems smarter to hang back and wait for the MMO to prove itself before making a commitment. This can be a little difficult when you see shiny new graphics and your friends are telling you how cool the battle animations are etc. There is also the fact that you don't want your character to be 6 months behind everyone else's.
Problem no 3: MMOs always release full of bugs and design issues. This does nothing to bolster confidence when determining whether to make a commitment.
Problem no 4: MMO companies love to slap you with that $15 US credit card fee right off the bat, before you've barely tested the water.
This always pisses me off, especially when I've just paid $50+ US for the game. A reward would be nice, since I just choose to buy their game over many others out there... but nope... more money please.
Problem no 5: When the meter is ticking, it feels like you need to get your money's worth. This sux if you're a casual gamer or even if you want to play multiple MMOs at the same time.
What happens if I want to play a new MMO casually? Is such a thing sacrilege these days? I don't like the feeling that my money is draining away when I'm not playing...
Solution no 1: Don't charge a subscription fee.
Solution no 2: If you are going to charge a subscription fee, don't start doing it until players have made a considerable time commitment to the game, or at least felt like they've had their initial money's worth (like 3 months). It's harder to leave when you've invested more...
In summary, the MMO companies need to lose the subscription fee or their MMO will struggle greatly to get off the ground. I'm sure I'm not the only one having commitment issues.