I'll pitch in my $.05.
Dave IMHO was a master of improvisation. He understood how important props were, and really made the game a thinking environment.
For example, in a game I played in when he lived near Willits we found a Deck of Many Things. We had the choice of 7 or 8 tarot decks to pick from, and Dave adjudicated what happened from the cards drawn. I did pretty well until I drew a time jump. Unfortunately Dave died before the character was due to re-enter the Arduinian time stream, so effectively he's in Limbo ;-) The entire day was us blundering 'off-map' into one of the small forests in Arduin. Dave was pleased as we'd found a 'new' site he'd had on the back shelf for awhile. It wasn't fully done, but I sure wouldn't have known it if Dave had not said so. He could move smoothly from a statted site into an ad libbed area and most of the time you were caught up in the dream he spun.
Carded items were another area he really pushed the boundary on. I know several folks who used carded items before meeting Dave, but the use of artwork and the quality pushed the state of the art forward. His box of scrolls, sealed with wax seals was a real eye opener. I still have 5 or 6 saved somewhere and wish I had the time to make a bunch up for use in my world. The background music also was a first for me. I still listen to Hawkwind because of Dave.
Dave could project personalities with a bit of accent and mannerisms. You could believe there were different NPC's instead of a bunch of cardboard cutouts being pushed around by a re-cycled wargamer. Dave had EPIC story arcs and lots of hidden clues.
While Dave borrowed ideas shamelessly, he added twists and made the result his own. The Cheshire Toad comes to mind... some of the monsters were far worse than the originators thought up. Dave's evolved Aliens were unspeakable! I truly believe he understood all fable is essentially shared material. Look at all the folk tales that have regional variants. Dave's genius could be said to have told then again in his unique setting.
I learned a critical lesson from Dave. Didn't matter the power of a player, from peasant to godly. Dave was in control and he could pull a vorpal rabbit from his hat and make you run for cover if you played like an idiot. If you played well he would challenge and entertain you. I'm proud to have died in a rear guard action in the Skull Tower one rainy day. Two of us died to let the other five in the party escape. Damn, we role played, not roll played. Sure I lost a character. So what! I had a great day and a tale to tell. Friends made it out of a no win situation. Best part was Dave looking over at myself (and the other Dave that day) and smiling..."didn't think you'd figure out how to get anyone out, good job." He was prepared to total kill the party as we'd gotten cocky/stupid and went too far.
I'll keep that praise in memory with fond thanks to a truly great DM.
I don't get to run games often anymore, but will still try for the bar Dave raised. May be rusty, may be distracted by life but that's no excuse. Dave could capture the imagination of a bunch of strangers and make dreams seem real while we played. That's a combination talent and gift that makes the memories live.
Hope others can fill in their memories.