Age of Conan, Darkfall and Champions Online to have a combat system where, for example, dodging an attack actually involves moving yourself out of harms way. The alternative to this action combat style is the system that most MMO gamers of the last decade will be very familiar with, seen in games like World of Warcraft, Lord of the Rings Online and countless other titles.
MMOs without action-based combat still have their share of mechanics to keep things interesting. Abilities that must be used from the back or the side (or are more effective that way) mean that combat isn't necessarily a stationary affair, and you can still hit multiple targets in a lot of cases. Some might even find that the action style is too "twitchy" for their liking. Proponents of action combat however might say that they'll never go back to boring auto-attacks. Is a real-time action combat system something that all new MMOs should aspire to have, or just another option that has its place but shouldn't take over?