I have come to a conclusion about what, for me at least, makes a game great. There are two components to every game that matter. The first is the story, the actually narrative that you play through. Now there are a good number of games that have a good story (though not that many that have a great story). It is for me, in a lot of cases, the narrative that draws me in and makes me want to continue.
The other factor is the actual gameplay mechanism. What I mean by this is the way the game is actually played. Be it a platform game, first person shooter or a racing game. Taking this at it's purest form, lets have a look at Bejewelled (or one of it's many clones). The gameplay mechanism is essentially matching lines of the same colour gems on a grid, to make them disappear. This is one of the simplest game mechanics, however can easily be as satisfying as building civilisations or blowing up space nazis.
I am sure you realise that a game like bejeweled is completely without a narrative, but it is still a great game. Hell Tetris, one off the most famous games ever made, is gaming at it's purest form. So clearly a good mechanism doesn't need a good story. One of my favourite shooters, Far Cry 2, has a very poor story, but that open world options and satisfying shooting makes up for it. On the polar opposite Dragon Age Origins and the Witcher 2 neither feature combat that was great, but had a fascinating story.
Of course some games manage to marry the two together, then you get something special. Half-life 2, the Uncharted series and Mass Effect spring to mind. I think it is these games that, if not necessarily being the best games, are the games that find a place in my heart.
I think that this is perhaps why I am no longer interested in most mmo's, the basic game mechanic are not that satisfying. A case of clicking on an enemy and then pressing a series of number keys. Wow that sounds boring, however it was more or less the same mechanic in both Dragon Age games, and I loved those. But the primary difference is that the Dragon Age games had a great story, something that is sorely missing from most mmo’s. Hell I was really enjoying WoW when I went back to it for Cataclysm, because they significantly bolstered the narrative behind the quests. However when I hit max level, I was back looking at the gameplay without the narrative, and hey presto I lost interest. Maybe I should try The Old Republic, maybe the stories it has can compensate for the “traditional” gameplay.