Observations, in rough order of significance.
1.) The default weapons are some of the most powerful, flexible weapons in the game. Rocket launcher, minigun, scattergun, butterfly knife, sticky bombs, medigun (etc.) are all core to game play, and you'll never be at an item-based disadvantage playing with a stock loadout. The new items (all the way back to the medic update), range from situational to strict downgrades. The only real mistake was version-1 backburner, when it gave 50 health, and that only lasted two months. (Melee weapons are the possible exception here--especially the ubersaw and equalizer--but it's probably okay balance-wise for the melee weapons to be above the curve.)
I played last night; some players had bought new item sets. I didn't really notice a difference. Sure, if you get hit with milk (non-milk substance!), another scout has a 1v1 advantage, but that's a corner case. The new sniper rifle is good, and makes the sniper more team-oriented, but that ignores just how absurd the default sniper rifle is. On the other hand, being killed by a fish IS mighty humiliating.
The upshot is, that you can compete on a level playing field without spending a penny at the store.
2.) The store is purely value-added. The new items are in the same mold as the community-created content added this spring. Valve could have continued to release them the same way, by adding them to the drop system. This time, though, the also provided the option (apparently a widely-requested one) to allow players to purchase the items instead of taking their chances with the drop system. The important thing, though, is that if you don't want to pay for additional content, NOTHING HAS CHANGED. From your p.o.v. the system is just like it was for every other set of new items since the sniper/spy update.
3.) The comments about "grinding" being a poor way to earn paid content pretty much entirely miss the point. Valve was wise enough to tie item drops to nothing but time played. There's no kill farming or achievement whoring required. If playing TF2 qualifies as "grinding" for you, I'd suggest that finding a game you enjoy.
4) TF2 players complain. About everything. Grenades (lack thereof), pyros, the Sandman, demomen, achievements, new weapons, hats, the drop system, idlers, should I keep going? It wouldn't surprise me if somewhere on the forum, someone is complaining that Red is OP. Lost in this is the fact that the game is incredibly fun, and has provided amazing long-term value. The game has never in it's life been 50 dollars (unless you count the Orange Box), and has never had a subscription fee. And yet Valve has quadrupled the number of available maps (not even counting the community-made ones that they incorporated and polished), tripled the number of available weapons, and added two entirely new game modes. Complaining that they began offering the option to purchase freely-available content three years into the game's lifespan is the very epitome of ingratitude.