An example of something that (I thought) needed to be re-edited: In an attempt to create suspense, the author withheld key information from the reader for about a hundred pages, so that when the details did come to light, it felt like the author re-wrote history. Either that, or the author was purposely manipulating the reader to believe one thing, while an entirely different thing was actually true. Sometimes that technique works -- but in this case, for me, it did not.
Still, even through my disappointment of these "weak spots," I'm able to gain some value in them. Reading any work through a writer's eye can always be a positive experience. Being able to spot weaknesses actually makes me a better writer. Because hopefully, by recognizing flaws in someone else's work, I can learn to spot the weaknesses in my own.
In fact, reading is such a vital learning experience that, whenever I'm consumed with grading freshman essays and don't have the creative energy to write, I always make time to READ. Because as much as I learn from well-written work (I like to study the craft, to see how they "do it"), I can also learn from poorly-written work (what not to do, which is equally important to know).