I'm worried about HL3
With one of the strengths of HL1 being the physical puzzles, they were x10 more awesome in HL2. Portal has stolen the mantle of physical-puzzle king from HL2, so valve will have to create some fantastic innovation again with physical puzzles just to keep to current form
HL1 being set in an underground facility, albiet a huge one, you could get away with physically small, linear maps that lacked open-ended outdoor spaces (likewise with portal). HL2 maps felt like outdoors corridors. In a game that rewards lateral thinking and imagination to solve physical puzzles in some places, you are forced into certain behavior in other areas (you can't just park the car/ hovercraft and climb over the cliffs. You can't go round the problem). HL3 will have to reconcile this somehow
Finally in both HL1 and Portal 1, you had the feeling of being the last lone survivor in a very bleak and hostile world. You saw so few humans that you often got the creepy sensation that maybe everyone else in the whole world really had died and you were the last person left. Incidentally Doom 3 and Bioshock also did this, (though I think the best way to achieve this effect is when very occasionally you'd find a single other soul or two, but always nervous and fearful and surviving only through their own physical isolation (locked in their secure lab but unable to get past the monsters at the door). Doom 3 and portal 1 had too few of these other survivors to really hit the sweet spot for me, and instead just felt lacking in content and realism).
Alyx, and later on, Wheatly in Portal 2 are irritating beyond belief, interrupting the flow of the game with banal chatter and failing to actually assist in any more than an artificial way in contrived situations that you had no control over, and that really felt like they were thrown in just to justify said character's existence. If HL3 has any Wheatleys or Alyx's in it, it will, for me at least, be fail
