What makes the code unmanageable ?

And so you keep writing good code and everybody else do, and then bah ! you look at (somebody else's) code and do feel that you would never want to touch that. The question is - what exactly makes the code such - horrible, scary, ugly and in general unmaintainable ?

To simplify, let's say we have a team that has set up the formatting, naming and other style rules beforehand. Will it save from ugly code ? No (but it undoubtedly helps). And so we have a group of professionals who have agreed on what a good code is, looking at the bad code they wrote.

If there is something wrong with the code itself (which basically means that all the members agree on its low quality, including the author), then the rules must have not covered everything and legal code is bad.

Now, given the code is not idiotic, it has indentation, theOnlyTrueNames, interfaces and whatnot, what about it turns others away ?