While MGO is part of MGS4 and can help the lasting appeal, I don't think MGO is that great. It was developed seperate from MGS4 I believe and Konami has been running it very poorly.
These are the problems I beleive it has.
Lag can be troublesome and gameplay feels weird (I can hardly kill people while others waste people, but these people could be the host so that could explain it), it doesn't look like it accomodates lots of people (each server can only hold like 1000 people I believe), and they're going to pound people with microtransactions (you get to make one character per console and if you want more, it'll cost you money. Heck, it doesn't even use PSN at all, you're forced to use their crappy KonamiID/GameID system which was a complete hell during the beta.
