So, did Bethesda ever implement the stuff they showed off with that Gamejam vid? Apparantly they let the devs run wild for a bit and came up with all sorts of neat stuff like spears, footprints, changing seasonal foliage, water arrows to make shit more thief-like and even mounted combat. Almost gave me hope for Bethesda. Don't recall any of the stuff actually made it into the game though.
EDIT: Found the old vid.
00:54 - Killcams for magic and bows were implemented in one of the patches (and so fucking awfully buggy that I had to remove both with mods to be able to use magic or bows without getting non-fatal killcams every third shot).
01:24 - Darkfall Cave added in Dawnguard had a water current in it.
01:46 - Adoption was added in Hearthfire, as was building your own home.
02:00 - "Goblins" were implemented as Rieklings in Dragonborn.
02:04 - The fast-travel waygates were implemented in a specific region added by Dawnguard.
02:08 - The EPIC new mount was added in Dawnguard.
02:11 - Mounted combat was added in patch 1.6.
02:15 - Dragon-riding was implemented in Dragonborn.
02:25 - Kinect shouts was implemented in a patch.
02:44 - The new water visuals was implemented in a patch, and for the first time ever removed that shitty Gamebryo-transition from above the surface to below the surface, where you could see right through the water all the way to the bottom.
02:54 - Crossbow bolts (although not arrows) with magical effects where introduced in Dawnguard.
02:58 - Werebears (only NPC's, not playable) were introduced in Dragonborn.
03:02 - Lycantropy perk tree was implemented in Dawnguard.
03:05 - Vampire melee feeding attack was implemented in Dawnguard.
03:09 - Vampire Lord form was implemented in Dawnguard.
There's also
a list on UESP that adds a couple of things I missed, but that list lies about the spears. The kind of spear seen in the video has never been implemented by Bethesda.