Let me be perfectly blunt: Half-life 2 is nearly perfectly executed. Were I able, I would rate HL2 a 9.5, but two things hold it back from being a "perfect" game.
The first is that it is a victim of its own success - to borrow from a concept I read recently called the "Uncanny Valley", HL2 has graphics, sound, and physics so close to life that the minor imperfects become more pronounced.
As an example, Alyx looks, acts, sounds, and behaves very close to a real person. Nothing in her demeanor makes you think she's artificial. But her hair doesn't move *quite* right. This stands out because everything else is so amazingly done.
The second is the ending - to describe my "complaint" without giving anything way, HL3 better be right around the corner.