There are some common threads, we could probably put together a poll.
*I need to buy all the books (or sub category there of) and try to get players (or myself) to use: This was 2e, and fortunately it was crunch, not the vast amount of setting stuff. We just didn't have it for 1E...it got pretty out control before the lesson was learned. Both in books, but also in game play as various changes were shove in. Which leads to...
*Massive and constant house ruling to the point were it is another game, and then keep going so it is another!
*Yes, the DMPC. In this same, very formative campaign, I had lots of authoritative and somewhat interesting NPCs that did give key information and guidance to the PCs, but I never crossed that line. Then I bring in this Viking skald (it was a fantasy earth) to do the same thing, but with the group while adventuring. I thought the character was very cool. It fell so flat, I could just tell from the players faces. (I find that NPCs with some kind of clear flaw or weakness can work well in cases like this. In one case, I had the lead NPC get himself killed in the first session, leaving the players effectively in the adventure).
*Downtime all the time. Same game. Playing weekly, and we would play out merchants and waiters and so on and on. Later on lots of interaction with henchman, staff and servants. But not too bad. Though it was really seeing it as a player that I got it--it was modern day, and we were at point of high tension, and the GM had us play out renting a car, for no reason. Years later, one of my old players joins a game that has been going for a while, and it happened to be a city visit after several dungeon session. Taverns, shopping, PC specific NPC interactions, for just about the whole session. It was really just bad luck, but he dropped the game.
*Telling someone how to play their character: Sometimes advise or coordination is needed, and it took me a while to get this one, but most of the time the DM and other players should just let someone do what they want, within the rules. Its important.
*Saying what the player character does: Even worse is describing what a character does that is not your character. Obviously if the players misses the session then it can be ok. Some transitioning in say a PBP or between sessions maybe ok (maybe) (between scenes, it really has to be summary of clear player intent). This is one lesson I got watching a friend GM and it can be very painful.
*Under preparation: Obvious, but worth noting. And still the biggest a problem I see at a lot of tables. Be ready to DM, and not just in an exactly planned detailed sequence. Its ok to gently restrict choices, or maybe not so gently if it makes sense in context, but you need to be ready and to react with something at least moderately amusing. [This also means be ready with the VTT, the relevant NPC stats, etc, etc]
Probably enough, for now.