Haven't started on Episode 2 yet, only started playing Episode 1 last night, but it is just more Half-Life 2, which is great if you like Half-Life, but I've never really been that bothered by it. It's a solid enough shooter, but I'd prefer it if they'd kept just having the one type of zombie, and have more human vs. combine fighting. But I'm not that far into it, so it might pick up.
Team Fortress 2 is, again, great if you like that sort of thing. I've gotten plenty of enjoyment out of it, and think it's got a great class system, albeit one that means most classes have one very specific thing they have to do to be effective at all.
Portal is where it's at. I've just stopped playing my first session of it, and I'm part way though level 17. It's really interesting so far, and likely one of the best puzzle games I've ever played... the puzzles are challenging enough that you feel happy once you've figured them out, but not so hard that you're stuck on them for that long. If there's a strong modding community, and I assume there will be, that's great, but I doubt I'm going to play the game past initially completing it once or twice, and won't play any mods that aren't ridiculously well recommended.
I really doubt they're going to add the Portal gun to Episode 3. From the sounds of it, the reason the game is so short is that they had to spend ages on each level making sure you couldn't break it using the gun... what it can do is very powerful, so it can't easily be moved to a regular game, where millions of things are going on at once, like Half-Life.
Still, maybe they could figure out a compromise, like how they *HALF-LIFE 2 SPOILERS, I GUESS* only have the Gravity Gun on 'full power' on small, simple, segments of the game. Perhaps they could change how the Portal Gun works slightly, so it's more feasible.