As a Grammarly user, the thing that's been most surprising is that there's no option for an API to integrate with other tools. I guess with the growth they've experienced, there's not a lot of pressure to expand it, but it seems like a world of opportunity. I'm sure there are some good reasons, like the editing experience or API abuse, but their tool simply isn't the best overall writing experience.

To be able to use Grammarly within Sublime text or other editors would be incredible. As it stands, because you're forced to copy and past content over into their editor, the workflow is the biggest drawback. It's really handy in textareas on the web, but I've struggled to integrate it into my writing workflow because of the copy/paste process. Writing mainly in Markdown doesn't make it any more elegant either.