This blog isn't just about why Portal 2 is good. It's about why Portal 2 is important. There are certain elements that should be examined and imitated by other developers, since very few other games currently do them as well as Portal 2 does.
Humor
Portal 2 is a funny game. In fact, it's one of the funniest games I've played. That's an accomplishment. Humor is so reliant on timing, so it's tough to create humor in a medium in which timing is out of the creator's control. And yet, Portal 2 pulls it off phenomenally, and I think other developers could learn from it.
Cross-Platform Play
Portal 2 allowed you to play its co-op mode across PC and PS3. That's pretty rare in games. Obviously, most games don't lend themselves to any sort of cross-platform play that works, but Valve has shown that it can work quite well in a cooperative mode. Again, it's the kind of thing that would at least be interesting for other developers to attempt.
A Good Sequel
There are generally three types of sequels.
1) The iterative more-of-the-same type sequels that people generally complain about (Call of Duty: Black Ops, Fallout New Vegas)
2) The complete redesign which often turns out great, but is really a sequel in name only (Grand Theft Auto 3, Fallout 3)
3) The sequel that fixes a flawed predecessor (Assassin's Creed 2, Mass Effect 2)
Portal 2 is none of these. It certainly isn't iterative, as it adds a ton of new gameplay features and includes co-op. It's not a total redesign, since it very much feels and plays like a Portal game should. And lastly, it doesn't have the benefit of having a flawed predecessor, since Portal was a pretty excellent game already. That's pretty impressive, if you think about it. Portal didn't leave a whole lot of room for improvement, and yet Portal 2 added a ton of stuff while still maintaining what we love about Portal.