I hate two extremes of bad DMing:
1) No matter how well the party works together, and how good their tactics are, they are facing a TPK because the opponents are just too powerful.
2) No matter how disorganized the party is and despite all lack of plan, they will *always* succeed in their attempts without anyone ever being at risk of character death.
The other thing I hate:
3) Over-controlling DM.
No matter how carefully you plot it out, you will not be able to capture the bad guy before the DM has pre-ordained that he can be caught. Or the DM changes your action because what you wanted to do would "ruin things". For example, one PBeM I was in (and subsequently left), I was playing a CG thief. I had just seen my fellow party members captured by a corrupt city guard, but I was able to get away. I decided I wanted to sneak my way across town to a temple I'd visited earlier in the day (trying to get info), claim sanctuary, and then figure out who needed a bribe, what the bribe should be, and how to get it to them in order to get my friends released. Instead, because the DM knew that the corrupt guard was going to have a "plot vital" conversation in the yard of the city's mayor's house (where the others were taken), my character was forced to sneak into said yard so she could overhear what the bad guys (who really weren't bad guys, they just didn't want to openly oppose the mayor) were plotting. Like there was any reason my PC should have suspected that they were going to stand out in the yard and talk loud enough to be overheard.
Couple instances like that, and I felt like the DM had 95% control over my PC's actions, so I just said "here, take the other 5%." I mean, really--need that much control, write a novel, don't DM.