Author's comments:
This is my second run through HL2 and it is better than my original in every conceivable way. That's right, there is a solid 40 minutes cut off via tricks, shortcuts, and outright optimization. The quality of both movies has also been improved, particularly the high version.

After my first run I found a couple breakthroughs, but most of the heavy lifting was done by others, mostly those at the HL2 Done Quick site (http://www.hl2dq.tk/) or in my SDA thread (before transferring to the former). The credits are at the end of this page for those of you who are curious (consider them spoilers if you haven't seen the run yet).

To review the problems with demo playback:

1. Alyx ice-skates everywhere she goes.
2. Friend and foe alike randomly assume a "scarecrow" stance.
3. Some posters do not appear on walls (e.g. the Breen poster near the beginning that Alyx comments on)
4. The higher your velocity as you jump, the more violently the screen shakes and distorts. So if you're about to lose your lunch you know the guy in the demo is going fast!
5. The "gun down" glitch after Nova Prospekt.

Added entertainment:

I attempted to be entertaining during the 'boring' parts of the game, mostly the dialogue (first 10 times it was interesting, then...). If nothing else, the parts where I play fetch with dog, talk/follow Alyx in Nova Prospekt, and the second turret gun sequence were all, I think, improved with regards to viewer pleasure. You'll see what I mean in Nova Prospekt, especially -- there's even an easter egg I'm sure will be lost on most people.