With most 3.5e classes I've been able come up with a few house rules to fix up classes where I thought they needed improvement. Even monks. Druids, however, just have so many things about them that irk me that I don't even bother.
I don't care too much for their spell list, which on the one hand is mostly mediocre-to-useless stuff, but at the same time most levels have a brokenly powerful must-have spell or two (entangle & plant growth being my two biggest problem-children).
They change shape as a natural ability, which is never as cool in RPG's as it is portrayed in books, comics, & movies. Anyone who's seen The Sword in the Stone probably remembers the shapeshifting duel between Merlin and Madam Mim, with each one trying to trump the other's latest form. If only it were remotely like that in D&D. Instead, players find the one shape that has the most optimal attributes and use it 90% of the time (brown bear anyone?).
They have a pet, and core classes with pets are annoying if the pet is standard rather than optional. Players effectively have two characters, and those players have a habit of taking more than twice as long to conduct their turns. In big groups, it's one more character the DM has to deal with. In small groups, EL gets thrown off by the fact that there's this extra combatant in the party, yet it doesn't earn XP for itself (it's not as if the druid isn't a viable combatant on his own). Worst of all, the druid's pet is a ridiculously disposable resource. He gets another one the very next day, at no cost to himself of any kind. Good role-playing mandates treating the companion like a friend, but the class's design encourages a more utilitarian approach where the druid exploits his pet for all its worth, especially if it looks like the final battle of the adventure. In fact, I've yet to see a druid's animal companion survive one. What really galls me is that in comparison to the animal companion's expendability, the designers saw fit to retain the harsh XP penalty and year-and-a-day waiting period in the event of a familiar's, even though familiars are largely ornamental compared to an animal companion.