I bought into Magic: The Gathering in a big way back in the early 90's some time after the release of the Revised edition. I was fairly impressed, not only by the game itself, but also by the marketing possibilities. The sheer number of cards, combined with the rarity model, was very compelling, and provided virtually infinite possibilities for play.
However, as Magic became even more popular, new expansions appeared with new rules. This meant that, even if you were happy with your older decks, if you happened to see a new card in play with some unknown property (eg landwalk), you were now at a huge disadvantage: you had new rules to learn, and new combos to grok. At first I thought, "Fine. They are making this one update. I'll learn this, and then I'll be okay from now on." And we all know where this went. The game became a moving target. At this point I bailed out.
I'm starting to feel this way about other games as well, such as Race for the Galaxy and Dominion. I understand that popular games are going to get expansions because they will sell. And I also understand the notion that "you only need to play the expansions you want". But this isn't always true. Someone drops Race on the table. It's already got all the cards for the first expansion shuffled into the deck. No one wants to pull out all those cards. With Dominion, it's more the case that those who have the first two expansions will want to "mix it up". So you have to know them all, or you are out of luck. I feel like the expansions have actually affected my ability and desire to play the games at all.
That being said, I love Pandemic: On the Brink and would play the base game no other way. I also have expansions for Carcassonne, Keythedral, Lord of the Rings, and Arkham Horror. But these cases don't feel like moving targets at all. At some point, a series of expansions starts to get out of hand, and the moving target transition occurs.
This is one of those rare posts where I prescribe no solution. I understand gamers who want more of what they love, and I understand publishers that want to sell more of what gamers are buying. But it certainly makes me feel left behind.