In reply to ambrosite.

You have to remember that Matt first introduced the idea of post formats to the WP community way back in 2005 (I believe) in a tutorial about building in the asides feature. This was a feature that was around long before WP, and especially before Tumblr. Matt's galleries were also a pretty popular feature.

What we had were theme authors who were building these features (e.g., asides, galleries, etc.) into their themes. However, there was absolutely no standardization from one theme to the next. For example, one theme would tell you to tag your posts as "gallery" while another would tell you to tag them as "photo-gallery" to get a certain look for gallery posts. As a user, that sucked if you had more than a handful of posts you had to re-tag when switching themes.

A big part of post formats was all about standardizing a feature that theme authors were already doing. Tumblr was really secondary to that.