Some people like to know all the exciting stuff first. They can't wait for Christmas day to open their presents and they can't bear not to know the big twists coming in their favourite comics or TV shows months beforehand.
Me, I'm more the sort that will give the boxes a shake and try to guess what's inside, but the last thing I want is for someone to tell me. So I avoid spoilers whenever I can. Unfortunately they are sometimes shoved in my face without my choice, and that really annoys me.
It's bad enough when some outside agency feels obliged to reveal all the good stuff for their own purposes, like TV stations running trailers that give away the big twist - I may hate them for it and plot to burn them down, but I can understand that their priority is to get people to watch the show. Once they are actually watching it, the PR machine is too busy screaming at you to watch the next thing to care about how they have screwed over the current one. But what's worst is when the guys producing the original feel the need to give away the ending. Like putting it on the cover of the comic.
The all time worst spoiler cover for me was the X-Men annual (I forget which one) where the story involved mysterious happenings and the big reveal of who was responsible didn't ocur until well into the story. Unfortunately he was prominently displayed on the cover, so the dramatic tension was nonexistant.
The latest issue of Manhunter isn't quite in that league, but cover featuring the old Blue Beetle and then not having him appear until the final page feels like a cheat. It's misleading, and does a disservice to an excellent comic that is worth reading for all kinds of reasons that have nothing to do with the cover.