Blizzard's newly announced Hearthstone online card game is neither new to Blizzard (which has had a paper card game for years) nor online games (SOE has had online card games playable in several of its MMO's for years). It is a natural fit for a company whose flagship MMO is increasingly a platform which hosts a range of distinct types of gameplay, as much as a single gaming product.
So I'm in when it comes to WoW's Pet Battles and 5-man content (and possibly its pick-up looking for raid, if/when I get that far). I'm lukewarm on its solo game and indifferent to its crafting/cosmetics. I've been out of its PVP (which itself comes in multiple different flavors) and structured raid games for years. I may or may not be in when Hearthstone arrives (though this, unlike pet battles, is playable a standalone product without a WoW subscription - I'd be very surprised if the two products don't have tight cross-promotion).
If you look back at the old school PVE MMO's, I'd suggest that there was effectively one type of gameplay - killing mobs in groups - with some variation to be had in terms of how big the group and what you might do (e.g. crafting) to gear up for said. With WoW - and other games that have tried to mirror the something-for-everyone approach with varying degrees of success - we increasingly have very different types of gameplay under one virtual roof, to the point where we're no longer playing the same game. Rather, Azeroth is a platform that you go to in order to launch off onto one of the various gameplay options - some of which coexist in the open world, others of which build on its lore, and some of which are just about some good competitive fun.
Are we increasingly playing in a world of Massively Multiplayer Online Gameplay Anthoglogies (MMOGA's to pick an acronym that's vaguely pronounceable) rather than virtual worlds or traditional MMORPG's?
P.S. Hat tip to Josh of the Game Diplomat blog and various podcasts for producing what is almost certainly the Internet's first Hearthstone podcast within days of its announcement. :)