The earliest games were all about reflexes. Look back at any classic game you care to name -- in the days of the arcades, skill was determined first, foremost, and only by how well you could react to changes in a split second. Combat in an MMO, by and large, is a very different creature. Due to issues of network latency and changes in design overall, most MMOs feature a slower pace and a greater emphasis on carefully planning out attacks. Certain games have entire classes devoted to a strategic building approach, such as the Warden of Lord of the Rings Online.
But reflexes haven't vanished from games by any stretch of the imagination. There are fights with very little chance for players to react, fights where the difference between the dead and the living is how fast one gets off a heal. Upcoming games such as TERA and All Points Bulletin promise to bring a more action-oriented approach to MMO combat, but is that something you want? While no game swings entirely to one side or the other, would you rather have combat leaning more on your reflexes or your ability to plan?