The problem with having all the choice within each class is that each class ends up feeling too similar.
Not really: you can make more choice available within the boundaries of your class, without any similarity among classes.
And grenades, outside of clan play, are nothing more than a spam weapon. Most players in any game throw grenades at the first sign of combat, with nothing more than the hope that it'll kill someone because that would be "cool". The fact that it takes so much luck to actually use grenades effectively is a testament to how poorly they integrate themselves into a game based around skill, accuracy, and teamwork.
I played mostly pubs, and I got to tell you, people knew to run away from a live grenade. So no, it's only luck if the other guy is not mindful. There was a fair bit of grenade spam, but most of the time I actually found these grenade exchanges to be quite fun, requiring fancy footwork to dance around.
Furthermore, the fact that you can "choose" to be a combat medic in TFC shows that TFC, not TF2, is the game in question that has homogenized classes. TF2 is making each class MORE unique, MORE defined, by making combat with each class appropriately difficult or easy, and giving each class distinct abilities.
Um, no. Combat medic is a *style* of playing medic. You still get Medic's weapons, medic's speed and regeneration. If a game cannot support multiple styles of play for the same class, it lacks depth.
Think of it like this - there's a huge sentry gun blocking an entrance to the enemy base. If all the classes can remove sentry guns, there's no incentive for one to be a class which is the "best" at taking them down.
But if only a couple classes are really able to take down sentries, then you need to make a choice - continue with your current class, unable to take down sentries, but be better-prepared for the combat afterwards? Or choose the class that can destroy the sentry, allowing for your team to get through, but making combat for you that much more difficult?
I remember playing some TF-like Quake mod (Weapons Factory?) where only one class could take down a sentry. The gameplay was super-frustrating, because in a pub game, you really couldn't count on some random guy of appropriate class to destroy the SG if he feels like prancing around inside your base instead.
The bottom line is, all classes should be able to take out a sentry with varying degrees of difficulty (or in cooperation). Otherwise, the game is unbalanced, and no amount of skill will get you to capture because you can't get past that SG. And no, I don't want to switch classes just to destroy the SG -- I want to stay who I am.