2. Ideally, DLC should be multiplayer-only.
Remember the mess a couple of weeks ago when the servers went down at BioWare and suddenly all non-pirates were locked out of their game of Dragon Age? That sucked. Servers going down should not impact a single-player game. Being unable to reach the internet should not impact a single-player game.
But if the DLC is part of the multiplayer portion of the game, no problem. If you can't reach the servers then you can't play anyway.
3. DLC should NOT be sold within the gameworld.
This should be obvious. Characters inside of a game should not refer to the online store, particularly if the game is serious in nature.
People get irritated with product placement in movies. You know, suddenly Bruce Willis decides to have a Coke, and he holds the can so we can see the label. Or James Bond drives around in some hot-rod and we're treated to a bunch of close-ups of the car's logo. Imagine how much worse it would be if Bruce Willis turned to the camera and asked the audience if they could like to buy a Coke right now. Because this is something that is happening in games already.
4. DLC shouldn't be integral to the experience.
This one is a little subjective, but basically you shouldn't need to purchase DLC to get a full experience. Lack of DLC shouldn't leave plot holes, loose ends, and the feeling that stuff is "missing" from the core game. Think of it like deleted scenes in a movie. The movie should still make sense and come to a satisfying conclusion without them.
Getting back to Portal 2 ...
Really, idiots? This is where you've decided to draw the line? Portal 2? See, Portal 2 is the first game in years to actually handle DLC properly. Where were you guys when Electronic Arts rolled out "Project Ten Dollar", where aspects of a single-player game would be locked to the original buyer and people buying the game used would need to fork over more money? Where were you when BioWare put a virtual DLC salesman inside of the gameworld? Where were you when Fallout: New Vegas started handing out overpowered cheat items to people who pre-ordered the game?
Portal 2 is the most innocuous use of DLC, ever. There is no reason to be upset over this. You have not made a principled stand with your childish review-bomb. You've only shown yourself to be spoiled, entitled brats who have no grasp of the core concepts involved here. If this protest had any sort of reason or coherence, then you would have aimed it at some other game.
You can hate Portal 2 if you like. Hate it for being only about 50% longer than the average shooter. Hate it for having fantastical pseudo-science setting instead of a gritty, ultra-brown motif. Hate it for being playful and witty instead of macho and serious. Hate it for having puzzles instead of murder as its core mechanic. Hate it because protagonist Chell is just a boring analytical Latina woman instead of an awesome white Ex-Navy SEAL dude with short brown hair. Hate it because it allows console and PC gamers to play together instead of maintaining the firm platform segregation that's made the gaming community such a joy these last few decades. Hate it for experimenting with new ideas instead of sticking to what already works and has been done elsewhere. Hate it because you're an indignant little rage-bot and you know deep down you don't deserve something this good. But don't hate Portal 2 for actually moving against the prevailing DLC trends. If you're on a crusade against DLC, start with EA. Start with BioWare. Start with Blizzard.
Start with figuring out exactly what you believe in, and then take your stand.
Shamus Young is a writer and programmer. He also really hates that one game you love.