You could say HL2 is weak, but I'd say it's just outdated in some regards. After setting a milestone, the competition simply closes up and HL2 has had a long time without a new addition. That's why the tension for Episode 3 grows.
Episode 1 and 2 could be considered "DLC" by today's wording, so they *are* in fact working on a big content update right now (hopefully). But yes, with their increasing number of franchises, it does seem like Valve is slowly abandoning their flagship. Maybe the pressure is too big, their own goals too high (<- best reason imo), the idea well dry (since HL is the only original series, the rest was pretty much always another one's idea upgraded to a full game. Possible exception is L4D if I remember the origin story correctly, but it outside of LAN Parties it seems about as "dead" as HL2 despite more regular updates) or other projects are just more appealing.
But there are still some things that HL2 does right that many games still don't get right today. I don't want to make a huge article out of this, but most of it has to do with the writing and interactive storytelling; go through the Developer's Commentary if you'd like to be reminded. I still find easter eggs and hidden pieces of the story despite multiple playthoughs.
HL2 always gets me over 100 fps, way more than any other Source Game I think, but I didn't write down the numbers.
P.S.
You can't really say the Source Engine is that old as it constantly gets updated. Sure, it is still the same somewhere at the core, but it's exactly this upgrading technique that I find simply awesome because it (potentially) improves multiple games at once - if of course Valve didn't add 30 bugs for every 10 they fix, but that's more of a general problem and less the Source Engine's fault. I really like the concept behind it.
