I think you somewhat answered your own question with "Bolland and Clarkson." Clarkson was an INSANELY bad signing, the guy has very obvious 3rd line talent, why would you give him a 1st line contract? It really makes you question his ability to evaluate talent AND his ability to negotiate, which are two very important points for a GM. The Clarkson trade wasn't too bad, but giving up a 2nd and two 4ths for essentially a 1 year rental, then not making the playoffs, does kind of suck.
Further on his negotiation talents, 8 years/$8 mil per year for Kessel and 7 years/$7 mil per year for Phaneuf aren't awful, but they're both very favourable for the players, not the Leafs. A lot of GMs manage to negotiate favourable contract, Nonis paid these guys pretty much the most they could possibly get.
The main thing, though, is that the team he put together has choked spectacularly two years in a row. The game 7 choke last year was insane, and the current 8 game pointless streak is pretty much just as insane.
I'm not sure that I fully agree we should fire him, he has made some good moves too (the Bernier trade, getting Raymond dirt cheap, Liles for Gleason, getting rid of Lombardi/Armstrong/Connolly/Komi), I'd probably give him one more year to prove he can turn us into a legit playoff team. However, I can definitely see why some fans want his head.