While the overall tone of this letter is really nice one thing bothers me. When you sum up the mentioned benefits for Yahoo! and Tumblr there's a great imbalance towards the acquirer. IMHO from the PR perspective it doesn't sound that good, especially that there's not much about the benefits for the users.

EDIT: you can compare how David Karp handled these news delivery, addressing the possible concerns of all parties: http://staff.tumblr.com/post/50902268806/news