Let's think about bad DMing decisions (while recognising that everybody makes mistakes). Three examples from my own gaming past:
1) I can't remember many of the details about the session, but it was a TSR-published adventure Planescape module and we, the players, had gone through some gate or other into the Grey Waste. The DM described our surroundings as being an endless and featureless plain, but with a couple of demonic winged gargoyle-like beings somewhere in the middle distance. We spent a long time debating what to do. We couldn't return through the gate we'd come to. The plain was featureless. The gargoyle-type beings seemed dangerous. Eventually we decided that since we'd be noticed sooner or later we might as well try to parlay with them. On approaching them, however, they mercilessly attacked us. It then became apparent that they were immune to non-magical weapons - they were abishai of some sort or other. We all died in short order. A bit flabbergasted, one of us asked the DM, "Exactly what were we supposed to do in that situation?" He told us we should have tried to sneak around them. It combined pixel-bitching with rank incoherence - how do you sneak around something in a featureless plain?
2) A d20 Modern game set in a kind of Mad Max world. A firefight broke out with some motorcycle gang in an abandoned town. My character was caught out in the open facing an enemy with a large SMG. It was clear that he was going to die. I was already resigned to the fact and thinking up what my next character would be. The DM rolled 'to hit' behind his screen...and promptly did a very poor impression of disappointment and announced the attacked had fumbled and dropped his weapon. My character had survived. The fudge was childishly obvious. I glanced around the other faces at the table and they glanced at me. None of us mentioned anything, but we all knew: in this campaign, death was going to be impossible.
3) A cyberpunk-type game, which was short-lived and I think run using some form of GURPS. The PCs had gathered together for a mission in some forest somewhere, raiding a secret radar station or somesuch installation for secret information. There was also a GMPC involved. It turned out that the GMPC was really good at breaking and entry and the rest of us were lacking in necessary skills, so it was agreed that the GMPC would go on the raid and the rest of us would wait. Cue half an hour of waiting around while the GM went through the rigmarole of playing out the entire raid, in his own mind, rolling dice behind his screen and apparently going through the entire event as if an actual PC was doing it, before finally announcing, "He comes back with the information". I think we were all fine with the GMPC doing the mission. But why on earth the GM couldn't have decided the outcome in 10 seconds or with one dice roll, I have no idea.
Good games are all alike; every bad game is bad in its own way. What do these three anecdotes have in common? What unifies them? I have a hard time thinking of a common thread. The first is simply bad communication, or maybe just fuzzy thinking: perhaps the guy concerned really thought he had made it clear it would be somehow possible to sneak round the abishai (he did smoke a lot of weed). The second is clearly a surfeit of niceness: the DM is an incredibly good and honourable person who was certainly trying not to hurt my feelings, but ended up, like most people with good intentions, causing a certain degree of harm. The third I think can be attributed to an overzealous concern with realism or system that you might say is common to players of games like GURPS. The GM wanted to be absolutely sure that the "correct" outcome was reached even though it unreasonably slowed everything down.
Good DMing is fairly easy to identify. Good communication with the players, enthusiasm, reasonable levels of prep, non-arbitrary decision-making, reasonableness, willingness to listen, imagination. Except insofar as you can simply provide a list of antonyms of characteristics of good DMing, is there an underlying, root cause of the bad? Is there a single thing that we can point to as the ultimate source of malpractice?