Classes
The modern 3E notion of classes differs quite a bit from previous editions'.
Classes facilitate the game as a group activity.
True. If everyone has a clearly defined role, each member of the group should get a chance to shine.
Also true. Many alternatives to D&D didn't value simplicity, and that's a shame.
Classes channel character creation creativity. They provide a templated starting point for you when you make a character. They're not a straightjacket -- they are a median point from which a creative player can deviate. If you want a character who grew up on the streets but secretly wants to learn the arts of magic, you can create a rogue character and eventually multiclass into a wizard. With multiclassing, and skill and feat selection, you can create whatever character you want -- classes don't stop you at all.
Monte explicitly states that "The Genius of D&D" isn't his doing, that it "has to do with the early days of the game's development, and nothing to do with 3rd Edition." Oddly then, his defense of classes rests on 3rd Edition's flexible classes and multiclassing.
D&D's reputation goes back to its 1st edition, and 1E clearly did have classes as straightjackets. Sure, you could make up a different backstory, but the difference between two 5th-level Fighters was negligible, and any two 5th-level Thieves had the same skills with the same emphases.
Even in 3E, many, many classes are surprisingly inflexible. Couldn't every class have used Bonus Feats instead of set Special Ability progressions?
One serious issue with classes (and levels), already mentioned, is the inflexibility. Another, especially now that we have easy access to multiclassing, is that the classes don't often encapsulate just related abilities. That warrior training grants extra Hit Points and an improved BAB surprises no one -- but
any training grants extra Hit Points and improved BAB in D&D. High-level scribes and wizards fight dramatically better than their low-level counterparts, even if they aren't supposed to be great adventurers but just great scholars.
At any rate, the problem isn't classes so much as how classes have been implemented so far.