here is one little thing that's been on my mind lately that i know some people don't think about to much but it does make a difference in a raiding environment and even solo pve and especially pvp.
default health bar frames... usually on the top left of the screen is yours, and to the right of that is what ever target you have selected. to this day, it still bugs me every now and then (especially when i see it on youtube) is people playing with them being completely empty and just showing nothing but a mere green/blue/red bar as apposed to showing either "remaining # / total #" or the percentage remaining of health, mana, rage, ect....
personally my preference is to show the numbers rather then % (i know there are addons that can replace those frames and show both). this way when tanking i can always see exactly how much health i have as well as exactly how much mana i have (which does make timing regen stuff easier). or especially on a dk tank when i always want to have 20+ runic power, so if i do an ability that costs 40 runic power, i must have 60, so i need to know exactly how much runic power i currently have and not just look at a light blue bar and guess based on how big it is...
in raiding knowing % is also crucial in many cases where things happen at a certain % like phase change, or frenzy mode.
for those kind of bosses i still have raw numbers on, but i go into dbm for that specific boss and check "show boss health frame" which puts a tiny, drag and drop transparent frame on the screen that shows the % of the bosses hp remaining (also sometimes has a 2nd % on it like deathbringers blood power). doing that with dbm is also good if your not using a point and click healing addon (like healbot or grid with clique) as at that point you will have your heal target selected instead of the boss.
but even just merely seeing the bosses hit points is good to have so that way you can see the difference between bosses like marrowgar having ~22mil vs festergut with ~40mil (25 man normal). or also with trash mobs, being able to see stuff like death bound wards having roughly 4x more health then other trash is going to make a difference in your approach to them. or also seeing how some adds are easily killable while others like rotfaces ooze have little less that half the hp of the boss can help.
even with healing, i wouldn't want to go with out seeing the hit points as it determines the kind of heal the party/raid member needs. such as a mage, warrior and tank could have an equal sized chunk of hp missing, but the mage only has 28k hp total, warrior around 38k, and tank sitting at 67k. as you could imagine each of these players with 1/3 of there hp missing are going to need a completely different amount of healing and being able to see 18.6k/28k vs 44.6k/67k on the fly is going to make a big difference on what heal is casted on them or how many times heals are casted on them vs just guessing (although to be honest this is also why i really like healbot or grid saying how much hp each person has lost)
in pvp, i couldn't imagine not being able to see the hit points of an enemy just by clicking there toons body. it can make a big difference in your decision and a warlock with 15k hp is going to get killed in 2 hits cause he is obviously a fresh 80 in green/blues, but one with 30k is obviously pvp geared and going to be a lot harder to kill. or even with healers, one with 30k mana and 20k hp is pve geared (often disc priest or hpally with those stats) and going to be easily killed while one with 28k hp and 20k mana (pvp geared) can kinda run around and /laugh at you for a while.
one last note is if someone does know of a simple addon, that maintains the original look and feel of you and your targets frame at the top of the screen while showing both hp # and % value at the same time rather then 1 or the other then please do link it.