Well, my take on the topic...
I will first state I think the people trolling Metacritic to lower the game's review score are being really immature. The game is obviously not THAT bad. However, I think people do need to seriously consider the arguments made about how this sort of "unlocked costume" happened inside the gameplay originally.
The funny thing is, I have seen cosmetic DLC that hasn't bothered me at all. I ended up concluding that the differences I saw in Portal 2 are that...
A. Portal 2 is a full $50 game
B. The DLC was available release-day.
Killing Floor had cosmetic items (character skins) for a cheap amount, but I found that it didn't rile me because the game itself was pretty cheap - $20. If you decide you must have all character skins for the complete experience, that's about $30 in total.
Some free to play games make their money through cosmetics. Obviously, the low boundary for entry is $0. Then, if you are really obsessed about character customization, it may go up to $40...or even higher if it's the type of game with "time limit items" that you must pay for again. To some people, it's worth it to pay hundreds of dollars. To others, it's only worth it to pay $10. You get a wide boundary.
For Portal 2, the cost starts at $50 and only goes upwards, I think totalling $130 for everything they made. From an economic standpoint, that tells me Valve believes they've made a product that, in total, is worth $130. What I'm annoyed about is how they don't have that "lower boundary" the previous scenarios have, when people think it isn't worth quite as much.
Finally, there's the point of exactly what "cosmetic" means. I'm a choreographer for Black Mesa. What if we released the whole game with purple-black checkerboard textures, no voice acting, no music, no NPC scenes, but perfectly functional gameplay...technically? You'd be able to play the game start to finish, no problem, but it would look like shit. You would obviously want it to be spiced up. We then of course introduce the Black Mesa Gift Shop, where you can pay up to $100 to get the full experience.
I'm sure the above scenario sounds ridiculous and cheap to you. You now know what the situation looks like to someone who DOES care about extra cosmetics. If they were "worthless to everyone" as so many claim, then there wouldn't be anyone paying money for them.
I believe in DLC in theory, but there is proper execution to it. I certainly don't agree with the execution present in Dragon Age or other games. Either have it as part of the business model from the beginning (meaning you release for less than $50, unless you hold your work in very, very high regard) or you develop it as an expansion once you've finished (like Fallout DLC)