The experience, that is, not the book itself. The beginning was slow; the characters had it too easy, worried about the wrong things, were too passive, and were unlikeable at times; there was lots of telling and summarizing; and of course it's way too long at 177,000 words. And yes, I queried it around back in the day (probably about a decade ago). I did get a few partial requests and one full from an editor who was at Tor at the time. He never formally rejected it, but I bet my manuscript has long been recycled. Unfortunately, he did tell me at a convention that he would send me a critique of my story, but he never did. I'd like to think if he had, it would have helped me, not crushed me.
The writing wasn't terrible, though it could be improved. The adverbs made me cringe, but the setting and description worked. I'm still amazed I was able to keep seven different POVs straight. That's harder than it looks!
The best part of the experience was being able to see the craft issues that kept this story grounded. When I finish some of my current projects, I'd like to go back to this world and characters to reboot the story. And this time, there'll be more tension and action and less internal monologue. (Cackles evilly.)
When was the last time you revisited something you wrote earlier? Do you think you've improved more with the story-telling aspects of writing or the word-and-sentence part?