A situational combat command system, like the one in Dead Rising, could be a good way of making environments more meaningful and creating situational tactical opportunities for MMO players. These systems can increase a game's replay value immensely.
Dead Rising has a system in which the same button combination (X and A) initiates a different command based on where the character is relative to the NPC you're attacking. If you're behind the enemy, in front of the enemy, or beside the enemy, you perform different attack moves even though you're pressing the same buttons on your controller.
Perhaps something exactly like that is too much for your average MMO because, requiring a lot of information to passed between the server and client, it would be unforgiving to bad connections and latency. But the general principle could be applied to the character's environment without much difficulty.
For example, if a druid is casting his lightning spell in a forest of pine trees, he might see an addtional effect to his spell. In real life, if lightning strikes a pine tree, the sap often crystallizes and rockets out around the tree like bullets. That could be made to happen in the game: lightning strikes the tree, causing a small explosion and doing damage to all nearby characters. Perhaps the druid would have to be wary of using this spell in such a forest, lest he damages his allies, as well as foes.
Another example might be a warrior performing a particular weapon swing while fighting amidst a field of ant mounds. By targeting one of the mounds near the enemy, he would knock ants onto the foe, resulting in a damage-over-time attack.
Or a healer might use the mist off a nearby waterfall for a special area-of-effect heal spell. A mage might use the same mist to add a buff to his summoned water elemental.
All of this could be done with minimal stress on the server, because the player is merely targeting an environmental object like they would normally target an enemy. The objects could be programmed to always be active but only respond to particular skills/spells, which only particular classes would have access to. Thereby different classes would have special opportunities to shine in particular environments, and players would be rewarded for being knowledgable and perceptive enough to use those environmental opportunities. There are other ways to allow players to target such objects as well, like the "Ctrl" key function in Command & Conquer.