So I've been spending my time playing League of Legends and waiting for Guild Wars 2.
It looks like GW2 has really upped the ante on casual-friendly games. While I have advocated for offline progression, it seems to me an equitable way of addressing the advancement issue is to provide instant advancement for the PvP side of the game, as well as offering a truly free-to-play game (without cash shop advantages).
One of my main gripes about the current MMOG monthly fee system is that casuals, who spend far less time in-game, pay the same amount as hardcore players, who spend much, much more time in the game. Casuals just can't compete with that, and will naturally feel short-shrifted because they get much less game for the same $1 spent. If there is no monthly fee (and no cash shop advantage, for the cash-poor), then this disparity is a little more easy to take - especially if I can gin up a top-level character and join in PvP.
I still think there should be a whole set of alternative advancement pathways that advance 24/7, such as I've written about before, so that you always have the sense that your character is "doing something" even if you cannot contribute as much time as you'd like. Those alternative advancement lines don't have to have anything to do with combat or normal character level - they can be entirely side venues, affecting aesthetics, cosmetics, styles, lore, information, diplomacy, reputation, etc.
Another casual-friendly feature will be the dynamic event grouping mechanism, which basically allows any and all comers to join in and get equal distribution of loot for their significant contribution. Since a formal pick-up grouping for casuals can be a nightmare (usually ending with a teenager vomiting up a sailor-speak rant about your pathetic combat-role skills), being able to just jump in and not be responsible for playing like a pro in my role is a good thing.
Which brings me to the classes of GW2 - but, I'll save that for another post.