I had a chance to play through the game yesterday at a friends (took us four hours between us).
*Spoilers* If there was new material (aside from the Stalkers), I missed it. The plot was barely furthered, I was expecting some revelation regarding the G-Man, but he's attacked/blocked/captured by Vortigaunts at the start and never seen or mentioned again. You see the Crab and Mortar Synths at the end of HL2 which were stated to be in this installment, but I didn't see them anywhere, much less fight them. There really wasn't anything new brought to the party, just more of the same sort of levels. That's why I think Episode Two looks better, new environment (wide open forests), new enemies, new effects and from the looks of the trailer, a major plot element involving Alyx.*/Spoilers*.
It's more of the same Half-Life 2, which is a good thing, but meh, I was expecting a little more innovation. Remember how Opposing Force gave you the Barnacle as a weapon/tool? Or how you could use the Displacer to flip between Earth and Xen at will?