When you were talking about deciding whether a movie is good or bad, the first thing that came to mind was... but then you even mentioned it: zombie movies! Way back, when I discovered films like Evil Dead as a teenager, my friends and I all agreed: that movie was simply abysmal, on so many levels! Still, it didn't stop us from watching it again and again. There was clearly something about it we found highly entertaining. Eventually it drove us to seek out other such cheap horror movies with predictable story lines, fake-looking monsters, and hopeless attempts at acting. Later we discovered we were not alone: ever since Ed Wood people have been enjoying certain films specifically for being so "B". I would even dare to speculate that modern movies of the zombie-horror genre try to fit this profile, as I've heard / read the same 'wisdom' numerous times: "Those movies are good precisely because they're so bad."