It's a question of perspective, and what level the game was calibrated for. If the game is calibrated for 1st level, then yes it seems the 30th level characters will be challenged. The way I look at it, low to mid heroic level characters have an easier time hitting things because they can't deal with as much retalitation from monsters if they miss too much. At epic levels, the characters have plenty of resources, and can withstand a good bit of punishment should they miss more frequently.
I see the disparity as help for the low levels instead of punishment for the high levels.
As for defenses, most characters seem to have a weakness somewhere. And higher level monsters almost auto-hit those defenses. Wizards are the biggest exception to this (especially human wizards), but they pay for it by falling behind the rest of the party on hit points. Overall though, having one weak defense can create some un-fun situations if the DM decides to abuse it by constantly targeting those defenses to immobilize, restrain or otherise cripple the characters. A restrained ranger who can't shoot without provoking attacks, and can only do so with a penalty, or a perpetually immobilized fighter who can't get into melee, should not be the norm.