With a number of premium themes becoming available later this year, WordPress.com blog owners will initially be able to install one of two premium themes: Headlines by WooThemes and Shelf by The Theme Foundry. The availability of such themes will enable blog owners to stand out from the millions of other websites on the service, that is of course when more themes become available.
That is great news for WordPress.com users — I really think that you are better off going with a WordPress.com blog right now than you are going with Tumblr — BUT — I think this is only true if you are creating a ton of original content and want to be able to expand the blog in the future. If you just want to post funny pictures of cats and repost quotes you find on other blogs then Tumblr is the way to go. ((I do realize that my personal blog is on Tumblr still. I also still like Tumblr, but I think it is not the platform you should consider if writing is your thing.))