I feel that DLC like this (really, it's microtransactions to me) is perfectly fine. Nothing was held back from the game, it's no way incomplete without it, and it's purely cosmetic so your robot can look different in multiplayer if you'd like. Would people be making this huge a fuss if there weren't any at all? Would people be going "OMG, I can't believe there's no way for me to change the appearance of my avatar in multiplayer"? I highly doubt it, so why are people going "OMG, they're charging for completely useless, comsetic changes that don't affect gameplay at all"?
I'll give you another one, Mortal Kombat. People are bitching that they can't get all the bonus classic costumes. I actually read a post where someone said that people were having to buy multiple copies of the game to get all the pre-order bonuses.
The problem is that it was obvious and clearly stated the costumes were bonuses for pre-ordering from a certain store. There were also codes exclusive to the DVD movie and the collector's edition. These costumes (and fatalities) are simply cosmetic changes to characters already on the disc and don't affect the game in any way, shape, or form. They're simply there to either give you a nice little bonus from buying from your favorite store or to make you shop somewhere else if you wanted the costume for your favorite character.
In fact, I'd be surprised if the costumes weren't at the very least offered as cheap DLC later at some point. Or even possibly as unlockable in-game content that no one's discovered yet (or at least not as of when I last cared to check, haven't in a week or so). If people are buying multiple copies of the game (or buying the movie if they wouldn't have anyway) just to get pre-order bonuses, that's an indication that those people have bigger issues, especially since Tobias has come out and said that people shouldn't waste their money (speaking of buying multiple copies or buying the codes on ebay for up to $40).
Now if they were offering new characters as DLC already, then yes, I'd have a problem with that, but it's another case of purely cosmetic changes that don't affect the gameplay at all. And in Portal's case, aren't a lot of the items in the shop available either through having them in TF2 or unlocking them in game via achievements? Pretty sure I read that on some forum or another.
My next case is Star Trek Online. It has a cash store with microtransactions (much closer to the WoW reference, but hear me out please). for those of you that haven't played, every 10 levels (11, 21, 31, etc) you gain a new rank and access to a new tier of ships and equipment. You get one free new ship when you make rank and can purchase any others for in-game currency.
Then there's the microtransaction ships. These can be purchased in the cash store and are different from the other ships. Like the rest of the ships, they have their own stats and layouts and are unique. So yes, while these ships do change the balance of the game, they can also be obtained in-game via an alternate currency. True, buying them in the store is easier (plus it unlocks it for all characters on your account), but they are also available in-game for those who either have more time to spend getting the marks or who just don't want to spend real money for them. In my case, since I have limited playtime to enjoy, I'd rather spend it flying the ships I want to mess around with than grinding to get them. Plus now my marks can go towards some of the other ships I looked at but didn't think were worth the money to buy.
They also have micrtransaction costumes (for both your avatar and your ship). When I got my new ship and went to customize it, I didn't really like the look of any of the parts available to me and looked at one skin that was available in the cash store. I ended up liking the way it looked best and spent the money to get it (less than $2). That doesn't affect gameplay at all as my stats don't change, just the look of my ship and it was completely optional. And before anyone says they held back the ship skins to charge for it, iirc the ship skins weren't available at launch (though some avatar costumes were). And even if they were, you still had 3 skins to choose from already, if they wanted to make a little money off a purely cosmetic transaction, more power to them; no one is forcing you to buy the skins you know.
Sorry to rant for so long, didn't realize I had this much to write; but between the P2 and MK rants (and sometimes outright hatred) I've been seeing, I just had to finally say something.