I still think the HL2 world is much more coherent than the infinte-sized Black Mesa base. There are less monsters, that's true, but it makes each encounter more important than in the previous game.
About the difficulty, I dont think HL2 is as easy compared to HL1 as some of you are saying. If you have played through Hl1, you obviously become a bit expert in FPS of that type. (I remember that Tomb Raider Chronicles had interesting puzzles, but they were very predictable if you had played the previous Tomb Raider games).
I had much more trouble killing the hordes of combine in Nova Prospekt than killing grunts in HL1. The hardest part of Hl1 are the Xen jumping puzzles and maybe Nihilanth.
And last but not least, some people are talking about "feeling". I dont remember a part in HL1 that has as much emotional depth and importance than the part in HL2 where you exit the station and look at the huge Citadel, Breen on TV, troops and flying-spy-cameras.
Of course, there is no comparison between the xen levels and the super-grav-gun fights in the Citadel.
Some people are probably judging HL1 by things they have played in Op4 and Blue-shift, which are expansions. Wait for the HL2 expansions and then we can look at the global picture.
P.S HL2 was good, and much better than Doom3 or Unreal2. And much more engaging than FarCry from my POV
Long Live HL (1&2) !!!