Same here - I dislike enemies with lists of immunities and resistances as long as your arm, almost all of which exist only to frustrate mages.
But what I hate with a passion is the ever-present gratuitous spell resistance. The game system has dodge, so they could fsck things up a bit for physical fighters as well, and their spell-like abilities and stuns could reasonably be made subject to spell resistance. But they aren't, even though dodge is nowhere near as bad for a fighter as spell resistance is for a (soloing) mage.
The problem with spell resistance is that elites and bosses tend to have a lot of it. Runs of three or four resisted spells in a row aren't particularly rare, and two resisted spells in a row occur in most battles at least once. Against bosses like Piotin and Jarvia - hard hitters with a hard-hitting entourage - a double spell resist almost always means certain death. Not to mention that something like Nightmare flies only if you can cast a chain of four to five spells without any of the spells being gratuitously resisted. Meaning that full success is so rare that you can mark the day red in your calendar (as yesterday against the spider queen, w00t).
Today I had to fight Piotin again, in the name of science. His troops were erased, his health was down to the tiniest sliver of red - about an Arcane Bolt's worth - and he was some twenty metres away. Ajira was at 100% HP. Four resisted spells later she was dead.
This could have been avoid with distribution shaping (like a 2d6 instead of the roughly similar 1d12), or by cheating as they did with stuns. If they can shorten the duration of repeat stuns to one fourth, then they could gently decrease the probability of outliers to avoid things like a bugger who resists four spells in a row on the strength of 5% spell resist. Not to mention the really big cheats, like instant auto-hit abilities. If you start casting Mind Blast when a wolf is ten, fifteen metres away then the beast can run the remaining distance, decide to overwhelm, and win.