Setting things on fire in-game is satisfying to a degree that is psychologically worrisome. The implementation, however, is important.
One thing I really enjoy in Team Fortress 2 is playing a Pyro. When you light someone up, they really are on fire, and it covers the entire character model. They yell for help, announcing that they are on fire. If there is water on the map, they may go diving off the edge. If you get a kill, the body will smolder on the ground. Nothing messes up an enemy charge like lighting up a few people, who instinctively go looking for health in a way that simple bullets cannot cause.
Contrast this with your MMO. What happens when you hit an enemy with an elemental effect? Most of the time, it seems, you can have an animation during the casting time and for a second or so later. You can have someone on fire from a half-dozen sources, and they just have a string of debuff/DoT icons. Stacking DoTs is still a good thing, but is a far cry from the panicked shouts of ogres.
Some do this a bit better, or do for certain effects. Some of the City of Heroes control graphics are great. Lock your enemy in a block of ice, boulder, or sphere of electricity. Have little ice cubes fall off a slowed target. Great. The Lord of the Rings Online™’s Red Maid gets a great graphic on her immobilize, with red specters rising from the bloodied waters to hold you in place. Does your game do anything like this well, where an ongoing effect has a highly satisfying ongoing visual? Let us know.
It would be nice to see them scale, from a low-level singe to a high-level pyre. I am not sure how well that works for all energy attacks. I can see frost scaling from a blue tint from a low-level slow to a giant ice cube. I would need to think about some of the others.