I've read through this thread, and I can say without a doubt I like the official box art better than what almost anyone in here has suggested, lemme explain.
To those saying Chell needs to be on there: Chell is nobody. Chell is a model, never referred to, never speaks, never has any personality or background explained. This game is not about her, it's about Portal puzzles, and how you get through them, it's about YOU. I never found out her name until months after my first several playthroughs, and still didn't care. She is there for the sole purpose of when you look through a portal, she acts as a marker for where you're standing. There is no relationship between Chell and GLaDOS, it's just GLaDOS and you. Even the Valve guys have spoken on this.
To those saying GLaDOS needs to be on there: Think about why GLaDOS wasn't on the first game's boxart. If we showed her, it would give away a good deal of the story. We don't know what to expect going in the first time. All we know are a Portal gun is involved. A commentating computer voice was at first a nice goofy touch, then turned into a serious threat when we're almost moidahd later on. When we finally come face to face with this machine, we stand there staring at it for a good moment, reacting in shock, disgust, curiousity, whatever. "So, you finally found me." Whoa, that's what you look like? As for the sequel, what if she alters her appearance? Don't wanna give that away. Heck, finding her in her destroyed state should be kept to when you experience it, therefore drawing a far more emotional reaction.
To that one guy saying cake needs to be on the cover: Are you serious? Go to your room and think about what you said.
Now, why the bots? Easy, what did they gush about so much with all the reveals and showoffs in the past? The Portal Bots. They have dinstinctive personalities, they have quirks, they are new. It's a sequel, put something new on there, they'll work fine. The bot jumping through the portals is actually quite close to the simplified version that they would have used for the sequel, you can refer to Maraveno's post on page 3 for the first game's box art. Can't have one bot without the other, so in goes the second. Portal 2, two bots, nice, classy, yes. Kinda like L4D's boxart with the great amount of puns they had in there. There's a lot going on in the picture, some depth to it, maybe you'll have to think even more with portals, durhur I made a funny.
Final thoughts, look forward to the co-op, that is something to be shared with others, the single player should be kept more intimate and experience worthy is what the box art conveys to me. Sides, perhaps the back of the box will have Chell and GLaDOS for all you nitpickers. Yes the simplified idea could have worked, but that would have been too close to the original's. And yes, in the end, all it is is a boxcover art. You're not gonna pay it much mind when you're in the game.