Not complete info though, I don't use desensitize that much.
1) You cannot see your full affects list.
2) The damage counters that you see will not be accurate. e.g. A dismembers might look like a mutilate.
3) Most of the time you can still see your current hp, its just the amount of damage being dealt to you becomes inaccurate. So if you look at your prompt you're still fine.
4) You get some damage redux, between 10 to 15% (which is why its so awesome)
Most classes with a short affects list will find the damage redux worth the drawbacks. For those who have a massively long affects list e.g. paladin, some mages, will have to pay close attention when using desensitize, especially in group fights where they face the threat of being dispelled, and need to put up the most important protections up as quickly as possible.