I don't know. I've seen an issue with almost every class in different situations. Fighters, once they get to certain levels, are much less effective in terms of doing damage to opponents than wizards, barbarians, or even clerics. On top of that, clerics can be just as effective in terms of defense, if they so choose. The fighter was mostly made insignificant by the crazy increase in SA damage for rogues and the powerful offensive spells of the cleric. A high level party is much more effective with two clerics or two rogues than a fighter and one of the others. Feats are great, but they just don't keep pace with the other classes' benefits.
Wizards have a long history of sitting certain combats out, in my experience. They often hang at the back of the battle and say "Okay guys, you take care of it." The fact that they run out of any magical options makes them less wizardly, at least for some of the encounters each day.
Rogues are great in most combats, but become useless in others, as has been mentioned above. The one thing that could affect this is the oft-overlooked Use Magic Device skill. A rogue carrying a wand of disrupt undead becomes much more usefull, let alone something more powerful.
Nothing hurts the player of a cleric more than having to convert that flamestrike that they've waited seven levels to be able to cast into a cure critical wounds. Those offensive spells are nice, but they don't get used as often as we'd like because they make way for the healing spells.
These are just the basic four, but I have seen plenty of similar examples for the other core classes. I know that the game isn't broken, and I don't think that any of these problems make any class unplayable (although the fighter is close at high levels). However, if 4E can fix some of these concerns so that every encounter is fun for every player/class, then why shouldn't they do so?