First off, I have to say that I love my critique partners. They're fantastic writers who are at once supportive and insightful. They read every manuscript I throw at them, and they always have something helpful to say. They're also fierce defenders of those manuscripts, so when I feel like chucking one over a cliff, they're always the ones who talk me down from the ledge.
But while my critique partners are great writers and have great editorial insights, they're still not editors. And the difference is profound. When Shauna first sent me notes, they literally knocked my socks off. (I'm using "literally" in its newest sense, which is actually equivalent to "metaphorically" or, in other words, "not literally.") Shauna was able to see things that no one else had seen, including six CPs (two of whom are now published or about to be) and two offering agents (who've sold scads of manuscripts between them), and she was able to communicate those things in a way that got my creative juices flowing. As I've probably already mentioned, I ended up rewriting more than half of Steve (and now that we're nearing the end of the revision process, I'd say that less than twenty percent of the original scenes made the final cut).
I'll be the first to admit that when I'm critiquing a manuscript, it's hard not to tell the writer to just write it how I would have written it. Some things are right or wrong grammatically, and some devices are better or worse from a storytelling point-of-view, but lots of things are just different, and when you're a writer yourself, it can be harder to discriminate between the two.
Now, do I still think I'm a fairly decent CP? Yes. Do I think that qualifies me to be an editor? Not necessarily. Obviously, I couldn't do for Steve what Shauna did for Steve (and not just because I was the one who wrote him). She saw the story's strengths and knew how to help me magnify them, but she also saw the story's weaknesses and knew how to help me fix them. Good editors don't trade your words for theirs; they help you tell the story you meant to tell in the first place.