So, I played the HL: Day One demo, and I found a few interesting things.
First off, some animation differences. The famous airlock guard had a trio of animations that differed from what shipped. At first I thought this was just an alternate standing_idle, but it also has 2 other sequences - halting_player and standing_looking. (His gun isn't quite on his holster, which may be why they changed it. Though I dunno why they didn't just use a higher quality version of the same pose.) There are 8 or 10 animations present in the barney model that just plain weren't used in the final version, not counting the unused ones that actually shipped. I kind of wish that Valve had kept them in - I can definitely see some of these having been used in mods if they were.
The hgrunt model, which has been presumably pared down to save on filesize, does not contain a shotgun submodel, balaclava, or cigar guy. There are a few unused unshipped animations, one of which is curiously reminiscent of the Sten gun. (due to the odd rotation and the arm movement I assume this was a prototype shotgun fire animation)
The bullsquid death animation is different - instead of collapsing and spreading out, it does a backflip and lands on its back. player.mdl has a gender submodel slot, featuring Gordon and Gina, and a weapon submodel slot, featuring a shotgun and backwards Mp5.
Scientists were apparently originally slated to feature in an aquatic area, with a tread, frontcrawlling, and 2 floating corpse animations. I guess this is from a cut section of the Icky area in Apprehension? Other notable things include a trio of animations for the scientist who built the gluon gun crawling out of a hidey hole and showing off his work.
The dead_osprey model, which if I recall correctly, was used in Opposing Force, actually originated in the HL1 beta. Looks like it was probably meant to be switched out when you killed the osprey, then play the crash animation when it hit the ground.
Headcrabs have two movement speeds, run, which is the one we're all familiar with, and walk, which I don't recall existing. (When I first saw it ingame I thought it was limping.)
I know that some of the model content, such as the spazhard animations for the zombies, were included in the HL SDK, but I really wish that all the animations made for HL1 were left in the files. But anyway, moving on. Let's get some map screenshots up in this bitch. The second opening in this room in Office Complex is gone, and the hallway to the shotgun room is much more straighforward and not full of random crates. The submap does not exist, rather being nearly the exact same thing in the main map with the exception of the texture used on the fence. (My shotgun is transparent due to an OpenGL error. Decals flickered a lot too. I guess my GPU doesn't support the older versions?) The hgrunt in the first map of WHG is replaced with a barney, and there are vents in the walls. Not sure why those were removed, as the final version of the map doesn't have any significantly larger or more complex bits to it to warrant having to remove geometry like that.
Okay, now for stuff I can't show in screenshots - mechanics!
Auto-aim is always on, with no menu option to disable it unless you turn off the crosshair altogether. A neat feature with this, however, is that your viewmodel shifts with the auto-aim crosshair position, similar to non-ADS aiming in console games such as Goldeneye and Timesplitters.
The glock has no alternate fire - seems that was added late in development. There's also a modern-style tactical reload on it - if you have ammo, reloading will give you 18 rounds to represent the one in the chamber, but otherwise you'll get the standard 17. This also explains the reason why mods built with the HL SDK code often "give" the pistol the ability to reload at any time - seems in the version of the code we got, the tactical reload was removed as in gold HL, but the check for it wasn't - so if there's less than 18 rounds, and there will always be without the tac reload, the pistol can be reloaded. This feature, though cut, is long ahead of its time, so the only reason I can think of to remove it is that playtesters were getting confused.
Frag grenades are a lot lighter and more bouncy, closer to HL2's grenades in the distances and angles you can pull off with them. Not sure why this was changed, as the heavy grenades we got were a lot less useful.
The Mp5 has the same higher rate of fire as the Uplink demo, and on picking it up you have the full 50 rounds, rather than 25.
env_beverage does not restore health. The benefits of hydration were apparently lost on Valve until near the end of development.
A mechanic that was not present was the beta implementation of the long-jump module - but Gina's different line in the Day One Hazard Course gives us some insights as to what it was. Apparently the long jump module, like the beta flashlight, was going to use suit power, and a great deal of it per jump. The LJM, instead of a backpack, was a special kind of battery for "short duration long jumping" - either some kind of limited time powerup, or a second counter that you'd need to have points in in addition to suit power. They probably changed this to the final version because it was too complex for players and programmers alike.
Finally, something a bit cumbersome was that, like Quake, there was a limit to how high you can look in any direction. Presumably this was just a hold-off from the engine code they got from ID that they didn't get around to changing, but having it there makes barnacles a lot harder to fight.
All in all, the Day One demo had a lot more insights to HL1's development than I thought it would, having always been given to understanding that it was just literally the first few chapters of the game. I wish a lot of the cut animation content was left in for modders to play with, and I don't understand why a couple of the mechanics and map features were changed between the demo and release, (I'm looking at you, tactical reload and bouncy grenades! Also the damn extra Office Complex map we apparently didn't even need!) but there were still some dregs of the "bad" HL1 beta that Valve cut for good reason left in there too.