It's the: wow I'm so smart for figuring this out momet
But really the game had a different approach to it, atmosphere and the story were nice, the humor was also spot on. Basically it was a FPS where you didn't have to shoot anybody, spill blood and see the American flag waving at the end (looking at you CoD, BF), plus it did require you to use your brain a bit which was really refreshing.
So as you see there is much you can do different with FPS (also Borderlands which I know you love)
I wouldn't call Portal FPS... more like FPP (P for Puzzler, if that even is a word).
And I like Borderlands for 3 different reason mainly:
I like the RPG part with the skills and that every "class" actually plays completely different and the weapons with stats that actually matter to the gameplay (different kinds of damage, mechanics, elemental effects).
I like the smooth game play itself. Borderlands just feels really nice with the movement and shooting itself.
I love the silly humour and the well-thought through characters.
So yes, there is innovation. I didn't deny that. But there is only so much you can change without making it actually a game of a different genre - like Portal - which doesn't have to be a bad thing.
I am not worried that Halflife 3 - if it isn't a hoax like some game sites already suspect - will be bland or boring. Valve always delivered as far as I am concerned. But discovering the shooter genre new? Nope - don't think that will happen. Some new gimmicks? Probably.