The original Trammel was an alternate version of the Ultima Online world that did not permit free-for-all PVP with the victor free to loot all of their ganking victims' stuff. History tells us that, finally given the option to vote with their feet, players overwhelmingly chose to go "ganking optional" and we have never seen FFA PVP with full looting in an AAA MMORPG since. I'm sure there are folks who were (and are) unhappy about the changes in the genre that resulted, but the numbers who were actually willing to cancel their subscriptions for lack of ganking were dwarfed by the numbers who were staying away from the unrestrained griefing.
I would argue that WoW's solo quest system was the second Trammel. There are many arguments that the shift towards "massively single player" gaming have hurt the genre, but we haven't seen forced grouping in an AAA MMORPG since. Again, the number of players who are actually willing to cancel their subscriptions were dwarfed by the number of people who were able to enter the genre for the first time because they were no longer required to play in chunks of time long enough to make looking for a group worthwhile.
If that's the case, the rise of the item store, downloadable content, not-so-micro-transactions, etc may be the third coming of Trammel. Personally, I believe that item shops create an inherent conflict of interest; the game developers have a financial incentive not to make design decisions that would hurt item sales, where previously their sole incentive is to create a game that people want to play. Unfortunately, like FFA PVP'ers and forced groupers of the past, my vote doesn't really matter.
I play three major MMORPG's: WoW, EQ2, and LOTRO. All now offer things that I would otherwise want to obtain via some form of added transaction. Where would I go if I did want to exercise my supposed right to "vote" against the RMT model? Even single player console games have paid downloadable content these days. Very few players will be willing to give up on gaming altogether, and those few lost purchases will be more than offset by the vast amounts of money players will happily spend in item shops.
With WoW finally on board, along with most recent and upcoming games, the pure subscription game is most likely dead. Item shops, like optional ganking and solo leveling, are now a standard feature of the AAA MMORPG.