During my limited tenancy as a web designer, there were a couple constant annoyances that were like thorns on my side:
Tumblr is a fantastically simple and fun blogging platform – exploring the different Tumblogs is a great deal of fun and you can easily lose hours there.
However, since Tumblr makes is so easy for anyone to add and reblog content, most the blogs are chock full of stuff (like that’s a bad thing, right?) and pretty hard to fully explore. In particular, look at the photoblogs – there are Tumblr users out there that easily posts 30 – 50 photos on a daily basis, and your typical photoblog enthusiast can easily racks up thousands of posts. With Tumblr’s default navigation, you’ll be forever just skimming the top of the iceberg, since only the most recent posts are always comes first.
chris from Line25 describes some web usability pitfalls that designers should avoid. some good points here, though a few of them are relatively minor.