Wizards are the ones people complain about. I actually think the druid is the biggest problem: full caster plus shape shifting melee monster. Yuck. Take that back to the 1E druid whose shapeshifting was mostly utility.You don't think the way this thread is set up implies that wizards are the biggest problem and more important to fix than other casters? If so, I mis-read the issue and I apologize.
Clerics would be a bigger problem, and they certainly have the potential to step on other niches, but they have a clearly defined role that means they have to reserve their power to heal and buff.