2) resigning Hamilton and inexplicably Prince
Aside from that (letting Okur go, investing too much in Stuckey), his missteps have been tiny. Dumars has them back on the right path. Monroe is a blue chip prospect. Drummond was the type of risky move you need. Jerebko, Maxiell, Bynum are the types of guys smart GMs take on.
The idea behind signing Rip Hamilton (and keeping Tayshaun and Sheed) was to rebuild around the Pistons remaining Champions and keeping their team's winning identity and defensive ethos rather than rebuilding from scratch.
It didn't work out but I don't have any real problem with the idea behind that. He had the cap flexibility required to make that type of plan work. Unfortunately, Dumars blew the money (B.Gordon, C.Villanueva) and the plan fell apart. Good idea, bad execution.
Misspending Detroit's cap flexibility on Ben Gordon and Charlie Villanueva was a very bad mistake but Joe Dumars will learn from that (hopefully).
One of the main differences between a good GM and a bad GM is that a good GM can admit his mistakes and will move to correct them. Everybody makes mistakes. All GMs make mistakes. It's a given.
The difference is that when a bad GM (Bryan Colangelo) makes mistakes, he will will refuse to admit his error and will instead just keep compounding those errors (Bargnani over and over and over ... and over and over).
Joe Dumars has always been man enough to admit his mistakes (as did Steve Kerr in Phoenix) and moved to correct them. That is why, in addition to his previous success, I would still have a lot of faith in Joe Dumars turning things around for Detroit over time.