Honestly, when you deconstruct both Half life and HL2 there wasn't anything amazingly unique about the games. They did the right things at the right point in time, but it's not as if those things were unique in and of themselves. The physics and graphics of HL2 were mindblowing at the time, but now physics is expected from games, and the graphics aren't exactly stellar by comparison to today's games. HL1 played out in a cutscene-less, continuously flowing world that hadn't really been attempted before, but again, is now the norm. What would people expect to see from a HL3? What's the big draw, the big thing that has people go "woah" when they see the game for the first time that the previous two had?
That's not quite correct. Both HL1 and (to a lesser extent) HL2 defined the genre. Sure, they might not seem particularly special in hindsight, but these games did things that nothing else was doing at the time. HL1 created a template and a baseline for thousands of games to come, just like Doom and Quake did before it.
Now as to what the big draw for HL3 is? Well it's a good question. My dumb theory that I don't really believe
is that Valve has already determined what to do with the HL series: hand it over to the community. They have always been big supporters of mods and indie games and have hired from those groups very frequently. They're also encouraging community development with the Steam Workshop and by allowing games like Black Mesa to be sold. Valve thinks that the next big thing in gaming (and HL) is that amateur teams will form and create games that big studios aren't able (or willing) to create. They themselves started behaving like a big studio and became hostile towards their fans in order to push their point harder. Unfortunately their tactics weren't working as well as they'd hoped, so when Marc Laidlaw left Valve they asked him to publish the "Epistle 3" document in an attempt to rustle some jimmies and get people working on fanmade HL games. And look - it worked. There's now at least two teams involved in making a game telling the story and several others remaking HL levels in modern engines. If they do well, maybe those teams, and others, can continue HL's legacy with community-driven, fan-made games, just as Valve intended.