I've spent a lot of time recently relating my version of what an editor does. Most specifically, lately, what they look for, and do while actually editing. How about the things they don't do.
A caveat once again, this is my opinion. I do not, and do not wish to, speak for other editors.
I won't say never, because that's just too absolute, but I certainly would say that I pay strict attention to not changing the author's voice. That's not POV, that's not grammar--the author's voice is something so undefinable that it defies accurate description, but is definately something you know when you see it. It's the way they talk. The way they relate the story, and the way their characters relate to each other. Even who their characters are, and act. Again, that's not to say the editor shouldn't tell the author when the character is acting 'out of character' for the way the author has them set up. Or even suggest to the author that perhaps the character is a jerk (when he/she shouldn't be). But the editor shouldn't heavy hand the way the author sees the story, or characters in it to any degree that completely changes the story, feel, or plots, so long as they make sense.
I know for a fact that there are some instances where editors do this. In fact, Agent Kristen, on her Pub Rants blog series on what to look for in contracts specifically mentions that an agent, (or author) should ask before signing the contract, what changes the editor might want. How drastic they will be. Because it will let the author know if the editor is on the same page with them story wise, or if they are going to be expected to completely re-write plot lines, or character attitudes, etc.
In my opinion, if the subplots, or plots don't work, the story should not have been accepted in the first place. At the very least, it should have been sent back for revisions before the contract was offered. In a less than perfect world, that's not always the way it happens, but unless it completely cripples the story I do believe such tinkerings should be avoided at all costs.