I am one of the few people who appear to be genuinely uninterested in a sequel for either franchise.
I first played any of the Half Life games with the release of the Orange Box, a product I was more interested in for Team Fortress 2 than its other components, and frankly I think that I dislike it more because of its failure to live up to the years of praise I had seen before I encountered it. Half Life 2 is, in my opinion, just another first person shooter which, while a rounded package itself, has been outdone in every aspect by later titles - even if not strictly all at the same time.
I find it hard to muster an opinion about Portal, it's like a book I've read and subsequently felt no desire to do so again. I don't dislike it but I really didn't find it a particularly memorable experience either. I couldn't pinpoint any one moment in Portal that felt "awesome" and found the conclusion of the game to be a massive anticlimax, although not unduly.
I think my main problem with both of these games is that I don't care for any of the characters whatsoever: I realise it's essentially heresy to speak poorly of Gordon Freeman but what has the Half Life series done, without the use of informed characteristics, to make him anything more than a pair of floating hands in an HEV suit?
While I'm sure I would, eventually, play a sequel to either of these games, I have no great clamouring to see them released.