I think that it's a good idea for me to clarify the way I think about "fluff" or "flavor." I play Magic: the Gathering, and on the cards there is text in italics. It's often a quote or something that's cool or makes reference to the story at play in the set of cards, but regardless of its content it has no mechanical significance whatsoever. So when I'm hearing about "flavor" being the difference between classes, I'm hearing that the only distinguishing characteristics between classes in a role are ones that have no mechanical implications, which is not accurate. Your choice between Fighter and Swordmage, or Paladin and Warden, or any other permutation you care to name about Defenders (in this case) is a mechanically significant one. The way they function, the powers they possess, and the differing options (particularly during character creation) can have effects in both combat situations and other areas of the game. Your choosing to play a Charisma Paladin over, say, a Fighter means you'll deal automatic damage to marked enemies that flee you, and you'll have some ranged powers at your disposal that might aid you, something a fighter lacks (unless he spends a lot of time throwing Javelins). Aside from that, your Chaladin will probably be the "face" of your party, managing Diplomatic interactions, or attempting to Bluff or Intimidate enemies. On the other hand, a Fighter would have more powerful Opportunity Attacks, as well as additional attacks on marked targets. He might not be able to do so repeatedly, but at times the Fighter can mark multiple targets, depending on what powers he is using. A Fighter will never be the face of your party, but he will be at home with Endurance or Athletics checks.
Now, I can't argue that when you choose a role, that locks you into a certain kind of play-style. If you roll a Striker, it doesn't matter if you go melee Rogue, charging Sorc, or archery Ranger, your primary mission as a party member in combat is to make sure things get super-duper dead. But how you accomplish that will vary, and I (at least) have found that the variance is significant.
One thing about 4e that does bother me slightly, though, is the fact that the powers system, which I generally like, essentially eliminates the differences that defined the Sorcerer and Wizard classes in 3.5