As they pulled you out of the oxygen tent, you asked for the latest party...
Enable user “tagging” or “mentions” in posts, like on Facebook, or Twitter, for that matter, where you can @ mention a friend’s name in a post, their username is auto-completed and hyperlinked to their blog, and they are notified in their activity feed/by email that you’ve mentioned them. I’d suggest only allowing one to tag a user who is following your blog to prevent spamming, or perhaps provide an option to allow non-followers to tag you.
Additionally, Tumblr could roll out “Verified” blogs for brands/organizations/famous figures who could also be tagged/mentioned by all, i.e. “Great article on Frank Gore’s training regimen in @sportsllustrated this week,” where @sportillustrated would auto-complete to a bolded/hyperlinked ‘sports illustrated.’ Think: Facebook Pages vs. Personal Profiles.
More Tumblr feedback here.
Gang Starr - Work [iTunes link]
V; - 1926 [iTunes link]