I had a recent brush with this sort of thing. We were told, up-front, that it was an undead intense game. We would be working for undead (in this case liches) against a menagerie of undead which were managed by vampires. I believe my case to be an outlier, but I'll explain it nonetheless.
Things we were NOT told:
-Turning doesn't work. He thinks it's silly, so it doesn't work.
-The undead all have a homebrew feat which lets them ignore immunities to their special attacks (energy drain, ability drain, level drain, etc.)
-The intelligent undead aren't actually undead. They all had some kind of homebrew template which give them the immunities and special attacks of the monster they're supposed to be, but they're not undead - just humanoids.
So, I show up with a Cleric gone into Prestige Paladin. I'm not particularly focused on killing the enemies, my sister has always been more into berserking cleave-them-in-twain style characters and I let her have it. She would be dealing plenty of damage on her own, and I'd supplement that where necessary. Our party consisted of Cleric, Warblade, Wizard-rogue hybrid (he had a Truedeath crystal to bypass the undead problem).
My character was built on
-buffing the human blender with DMM
-turning undead if there was a massive amount, to give us space to breathe
-control incoming damage
-Lay on Hands
The CHA-WIS split was taken care of by the Serenity feat from DMC.
My reasoning for Lay on Hands focus here was that I could 1) Heal the blender, who conveniently doesn't pay any attention to her AC or HP total and naturally draws a lot of aggro; 2) Lay on Hands, with feats/items, can take care of quite a few annoying status conditions and heal a significant amount of HP; and 3) When a heavy-hitter or boss-type enemy walks in, I can use Lay on Hands to put some respectable damage on it.
Now, I let my DM know my plans. I told him how I expected to slot into the combat routine. He was satisfied and relieved that I was taking a back-seat part-buffer role. (I talked with him a lot about character optimization, though he doesn't quite grasp it much of the time he is fun to bounce ideas off of when he's around)
So, the adventure is going smoothly. And then we hit our first combat. Mass low-CR undead, zombies and skeletons and the like. We're getting pretty crowded, the flanking bonuses are turning near-misses on the blender into hits. I use Turn Undead to make some room. Except I don't turn much of anything, and that's with a moderately good roll. This happens a couple of times before I stopping writing it off as "weird" and ask what's up. DM tells me "oh, the undead have been buffed up so that turning doesn't work very well." Okay, would have been nice to know, but I'll just use my left over turns to persist another buff and move on. Not a big loss, wasn't really banking on using it for turning to begin with.
When we reach the first undead with a special attack (negative levels), I put myself out front and tried to draw its attention. It worked, and it ignores my friends and comes to beat on my face. I did this because I invested in Soulfire armor, anticipating it blocking more abilities than I could shake a wand of Restoration at. In short order, I found out that it would be blocking nothing. Ever. Because every single undead, he told me, had the ability to bypass immunities. I was getting pretty annoyed at this point. That's a big monetary investment.
Once the first big, soaky bruiser came out (Vampire with class levels in I-don't-know-what) and I walked up to expend some of my Lay on Hands pool to help the party bring it down and instead healed the vampire, I was done.
I told my party that I was really low on spells. We took an in-game night of rest at the next natural breaking point, and I repaired the stat drain and level loss as I could. The next morning, I prepared a very different selection of spells. I buffed myself into a monster, and just facerolled the rest of the encounters. Once it was over, I went on to kill our quest giver.
We had a long talk with the DM. Things are better understood, now. Players don't like surprises like that any more than the DM likes to be surprised by Dragonwrought Kobolds or White Dragonspawn.
Tell your players if the adventure/campaign is going to have a thematic enemy. Chances are good that their character would know that their world is full of undead, or that Evil Organization X is breathing down their necks, or that they're going to be adventuring into the Nine Hells/the Abyss.
Tell your players about all of your house rules. When we sit down to a game of D&D 3.5, we have agreed to follow the rules as written unless otherwise stated. If you go around changing rules without telling anyone, you're breaking that agreement.
If you're using a custom setting, give them a bullet-point list of important facts of that setting. If you're using a published setting, tell them which one and if you're changing anything important about it (e.g. if you don't like that Warforged aren't property anymore in Eberron, and decide to instead undo the historical moment they gained freedom, you should tell someone before they roll up a warforged).
Tell them if you have a particular weakness at handling something as a DM. I, for instance, hate dealing with butt-loads of units in play. I ask my players to not take leadership and to not make a horde necromancer/summoner/hordificer. One of the guys in our group hates dealing with divination, so we don't use them when he DMs.
