An easy return policy would be nice. With Steam I have two weeks to get a full refund on any game that I've played less than 2 hours of, which has led to me trying out many games because of the low-risk involved in making a purchase.
To be honest, I'm not even sure what the PSN return policy is, but whatever it is, it hasn't been communicated to me well enough to make the assumption that I can safely buy a game that I may or may not want to keep until I've played.
Other than that, the massive discount given to full-year subs versus subbing for a single month at a time makes it unfavorable for consumers who don't use the service very much, which is the case for all subscription services. I don't see this changing anytime soon, but figured I would mention it since it's something that I've personally been bothered by.
I play mostly single player games, so I use the service primarily for discounts and free PSN titles, and overall I think it's a great service.