The reason TFC was so good was that they didn't constantly add new weapons and the weapon set was somewhat limited. If you took a 6 month break and came back, it was like riding a bike. It's impossible to keep up with all of the weapons in TF2 and IMHO it is unplayable unless you play nothing but it for the rest of your life. The second you stop playing, you might as well never play the game ever again. I hope Overwatch doesn't screw this concept up.
Yeah, it kinda degraded the experience that Valve originally set out to do. It used to be very stylistic and served as a game mechanic so each character had a very specific set of guns. The hard strengths and weaknesses were easy to tell in the most chaotic of gun fights. Now I don't mind some of the alternatives they had (like the backburner to the regular flamethrower) because it only slightly tweaked the original weapon sets. However, to completely change the play style of the character becomes a detriment to the game. There was no need to turn the demoman into a melee sword character or to give each character 10+ weapons to choose from for each of the primarys/secondaries/melee. That said, it still is a fun game but it definitely has lost some fundamentals as time passed. Just like any actively updating game, if you don't keep up means you will always be at a disadvantage.
Cosmetic things like hats and body parts don't bug me but I agree with you when it comes to the amount of weapons they add.
I don't think it's unplayable unless you play nothing else. I frequently take year long breaks from TF2 and find that the core gameplay has changed almost none. There are generally some new items that I don't quite understand, but it's pretty easy for me to gain a general understanding of what's up within a session.