I don't want to restrict the subreddit, for one. I have around 295k followers on Tumblr; I can't hand-approve all of them. Plus, restricting the sub would keep new people from finding it.

Plus what the other commenter said about formatting and poor workflow. The mods can just barely handle the current workload of reviewing ~30-40 posts per day, choosing 20 to post, and adding tags as needed. If we had to open an email, then open a link (and hope the link was functional and not malicious), then review the submission, then copy the submission, then open the "submit a post" dialogue, then paste, then format, then post... yeah, that's way too much.

There's an IFTTT workflow for sending Tumblr posts to Reddit, but unfortunately the API it uses doesn't preserve any formatting, so you end up with a giant text blob. I don't think it could handle image posts. If that worked, I'd happily start sending things to a subreddit.