I agree that the ending was disappointing. But I suppose we will just have to wait for HL3 for that. It was also a bit shorter than I would have expected; I too thought the portal at the end would take you to the Combine planet. However, I thought the gameplay was excellent - I particularly loved "We Don't Go To Ravenholm," (the zombie section with the insane priest), and "Follow Freeman" (the tense urban warfare section near the end). The designers did an excellent job at managing the tension level, without making gameplay too difficult. One of my major complaints about Far Cry was that it made me reload far too often, even on easy mode (and that you can't save except at checkpoints), but this was not the case with HL2.
Additionally, the Source Engine is easily the most advanced FPS engine around today. Although there does not seem to be any bump mapping in HL2 (??), the physics simulation easily beats anything else out there, and the water shader is fucking amazing. I can't wait to play with this engine a bit and see what the mod community will do with it.