The no. 1 reason tumbler sucks: 1 original content site for every 1000 reposting shitheaps.
Also, people using it for posting info that would be much better presented in some other way. I'm looking at you safariextensions.tumblr.com, your site is pretty much worthless thanks to the decision to use tumblr.
That's a community problem; if you subscribe to a Tumblr blog (tumblelog, microblog, etc.) like you would with any other blog, this wouldn't be a problem.
It's a Tumblr problem for Tumblr users.
Many people - not necessarily you - tend to forget that Tumblr is (also) a blogging platform when used outside of the - atrocious - Dashboard.
You wouldn't say the same about Blogger or Wordpress, for instance; at least it doesn't look like crap in Tumblr.
Tumblr has definitely created a trend and affected the blogscape with its laudable ease with which you can publish content online, and it certainly has its pros and cons. Most of the cons only exist if you focus on them.
Also, people using it for posting info that would be much better presented in some other way. I'm looking at you safariextensions.tumblr.com, your site is pretty much worthless thanks to the decision to use tumblr.