I'm actually looking forward to Portal 2 as much as or more than Ep3. I hate it when people say "There's no need for a sequel" about any great game, because more often than not the sequel is an excellent product with a great continuation of the story and great building off of the original game's mechanics, and I'm especially expecting such from Valve, who consistently make quality games.
Why are you looking forward to a game that will be pretty much more of the same just with another story craply tacked on?
And there is no need for a sequel, at all. Give me one good reason why there should be a Portal 2 and no Half-Life 2 Episode 3. And if your excuse is "because it's great" then don't bother.
EDIT: Not to mention that Valve has different teams to work on their different games. Left 4 Dead 2 and Portal 2 did absolutely nothing to Episode 3.
Well apparantly it did...
Episode 1 was released in 2006
Episode 2 was released in 2007
We're in 2010 and still not even an announcement as far as episode 3 is concerned. So Valve may have different teams, but they are completely incompetent in using them...