The one game I've encountered that continues to grow and change without engendering the sort of knee jerk response typical of many MMO players is Magic the Gathering. The lateral expansion and evolving designation of what constitutes the "standard" versus "extended" or "legacy" types of play allows the game to stay fresh by essentially remaking it every few months. While the basic game mechanics remain the same, and certain archetypes transcend the various expansion and rereleases, the cards that make up your Type 2 legal "White Weenie" deck will probably be completely different in a year from now. Still the game, at least in the short time I played it, managed to walk to the tight line of having familiar enough, overarching mechanics, while still constantly introducing and adapting itself to new ones over time.

I'm not sure if or how one would apply such a system to a typical MMORPG but I suspect that some of the MMOFPSs releasing in the near future might come close. Any game with a short or trivial progression to the "endgame" and a focus on PvP as the central activity would be a good candidate for a TCG analogue in the MMORPG/MMOFPS mold.