You know, I remember when they introduced the backburner.
Damn, that was one overpowered weapon. And it changed the way most people played the game.
You mean the weapon that (until very recently) didn't have any airblast, which is one of the Pyro's most useful attributes? Most people considered it a terrible weapon precisely because of that fact, since you were pretty much limited to playing ambush or being a W+M1 idiot while using it. Even now that it has an airblast, the ammo cost for it is so high that it's essentially an emergency-use feature. In most hands, the Degreaser + Axtinguisher combination is much more powerful.
The issue is that Valve started implementing content which violated the core design principles it used to build the game. 'Don't remove player control' is one example. 'Allow snap recognition of all classes (and their capabilities)' is another one that feels like it's really degraded.
As I said in the edit I slipped in above, I don't really see how never removing player control could ever be truly construed as a core tenet when there's always been at least one class, and sometimes two, that essentially do so as their fundamental mechanic. Again, I agree that Valve went too far with that particular decision, but I also think that they had a good reason for doing so, considering the Scout's shortcomings. I've switched away from the Sandman myself on occasion, but I always keep coming back to it, because it's something that helps me actually survive encounters with much more powerful classes.
As far as snap recognition goes, I've always taken that to mean the ability to tell classes apart at a glance based on their easily-identifiable silhouettes, and that hasn't really changed at all. I do understand where someone like Joshua is coming from regarding identifying weapons and their uses, but on the moment-by-moment level, the majority of weapons use the same fundamental mechanics as the stock weapons, so your reactions to them don't change all that much.
It's no coincidence that most of the genuinely screwy items have been those that violated these design tenets - the initial Sandman and the superbroken-era Natasha are good examples. There are other items which do alter the gameplay which I think are excellent, like the Sandvich, but which ended up being broken because of related changes - the Heavy no longer has any real reason to use the shotgun due to spinup time changes.
The best Heavy player I know (the guy's just about pro-level) uses the Shotgun exclusively, I'd assume because of the ridiculous amount of mobility he's able to pull off with it. I swear, the guy's harder to hit than most Scouts. Even with reduced spin-up time, an actively-firing Heavy is a very cumbersome class in terms of movement, so having a weapon that allows you to be more mobile is still advantageous. Plus, even in public servers, good Heavies usually wind up picking up a pocket Medic, so there's not as much incentive to carry around your own health pack.
TF2 was built to be an intuitive, transparent, high-agency team game which didn't require extraordinary amounts of poopsocking. While I don't think it's a bad game, I don't think it meets any of those standards any longer, and other games I play do.
On the other hand it is still quite frequently hilarious.
I would respectfully disagree, as I think it still fits most of that to at least some extent. The explosion of weapon options is a valid concern, and I think it's one that Valve would probably like to do more to solve, though there are at least some mitigating factors that they have been able to implement. (Giving the player access to all weapons in offline practice might be a good idea.) But while it may have shifted away from some of those tenets somewhat, I think that what it represents in terms of a developer's long-term commitment to actively expanding and experimenting with a product more than makes up for it. It's certainly the most engaging multiplayer experience I've ever found anywhere myself.