I did a post on this a while back. Sometimes I go through samples and can tell if a manuscript is at the editing stage or at the proofreading stage. And sometimes the author is asking for the wrong service. And then sometimes I have to write and tell them their book needs to be edited rather than proofread. That's tough to do because the price is twice as much.
Indie writers take a risk paying money to get their books in great shape. Without a publisher to do their editing and covers, they have to fork the money out of their own pockets not knowing whether or not the book will pay out the money put into it and eventually make a profit.
I know one thing. A book that doesn't have good editing may be loved by readers, but they will complain about editing. The reviews will reflect that and book browsers will be turned off by that.
So what makes an editor good? All four of us have experience editing novels, and I also have experience editing educational texts and some nonfiction. Editors love to edit. I chose this job because I love to read and I love to fix, polish and make a good story shine. Since I started this company, I've been able to cut back on content writing and spend my days immersed in awesome indie books. I haven't had one I didn't get into and fall in love with.
As I said in the post I linked to, if you don't know which service you need, editing or proofreading, feel free to send a sample and I'll give you my honest opinion. You'll be able to tell from my sample that I'm being straight with you.