Wow sounds like a lot of people play D&D. Makes me feel a little less nerdy...a little haha. I mainly play 3.5, to me it feels more like a separate game than a paper and dice MMO...but that's just me, though I have played a 4th edition campaign. My DM is a bit of a psycho and likes to push us and make us think very far outside the box. He also gave us sanity checks, hello cuthulu, but that was because some of the things he had in the campaign, like a scorpion nest made entirely out of dead bodies or a creature that is just a decapitated head and spine that would explode when it died, were horrifying and probably would drive someone insane. Our party was made entirely out of strikers (One sorcerer (me), two warlocks, and a ranger) and we didn't really do so bad. No healer was really tough though.
Some advice, no matter what system you are playing, make sure you can actually work with the people you are partying with. Whether you like them or not, in game or out, going it on your own is usually suicide and if you can't work with the party you have...your more than likely going to die. It's also good to know what your party can do so no one is caught off guard when something pops up that no one can handle. Also knowing what you all can do will make it much easier to work together because you wont end up stealing someone's role in the party and make them feel useless. I have been in a campaign where NO ONE got along with each other. The only reason we survived AT ALL was because of the amounts of damage we can do. But when a curve ball came along we almost died...a lot.
Also be able to adapt to what the party needs. Just because you are lawful good doesn't mean be a dick and constantly block the party, Aka lawful stupid. An example is the campaign I have been in for 2 years or so now. We are finally level 14, the DM gives out very little exp. The current campaign I am in is weird, basically we are the last heroes in a world where everyone is evil and wants to kill you. Everyone in the party is chaotic evil or neutral except for me and one other, the two healers. We are playing Favored Souls, 3.5, which is a really good class if you do not take anything else haha. Anyways, rather than freak out on the party when they do some really stupid and nasty things, my character has come to accept that the best way to save the world is to just let them do their thing and I will just keep them alive. As long as I do not know about some of the things, I'm fine. We are a four man party, one caster, one rogue/tank(hard to explain but his AC is in the 40's), and two healers (one archer(me) and one flanker(my friend) and we work soo well that the only thing we could really use is a Bard...if we actually decided to talk things through. As the healer I have barely had to do MUCH healing and when I do the party is completely fine.
Also like Tatile said, the DM should be able to handle anything your party throws out. In my campaign we actually killed the final boss three bosses before hand haha. He was soooo confused on how it worked but it did. Also later one I revived someone who by all means should be dead in the story. I had been saving up diamonds just in case the party needed to be resurrected, but I never used them. So when my DM said, "You see General So n So, who died of his wounds because he wouldn't accept healing." My character was so mad at his stupidity I looked at the DM and said, "How long has he been dead?" He said only a few minutes. So I walked up, slapped my hands on his chest, and used Raise Dead. My DM looked at me like I was crazy and said I couldn't, mainly cause it was story. Only answer..."I Just Did." I would have raised the second commander too but the stupid poly morphed dragon flew off with him to go look for a healer...EVEN THOUGH I WAS RIGHT THERE!
All in all. I always enjoy a good D&D campaign and I hope you find yourself a good one too because it is really fun. I sit around sometimes and just read the Spell Compendium when I am bored or build characters just in case another campaign comes along that I would like to jump into.