Okay, well, here's what's going on. The theme is coded to have the gradient to extend a certain percentage of the page. The longer the content is on the page, the longer the purple gradient will extend down the page.
I think in your case, the most recent posts were long enough to make this behavior unpleasant, something not experienced before.
My suggestions for the moment:
1. Limit the number of posts shown on the home page (Settings->Reading)
2. Use the "more" function to limit the amount of text that appears on the home page. ( details: http://en.support.wordpress.com/splitting-content/more-tag/ )
On the individual post pages, when there is a long entry, will suffer the same result too.
Frankly, this seems like a bug and will ask the staff to review it.
To the staff: The background gradient for the #main in the spectrum theme applies to the entire height of the content, thus changes based on the amount of content. With virtually no content, the gradient looks nice as a small splash of color above the site description.
With a fair amount of content, the effect reaches far enough down to make other elements look poor (ref: the OP's category listing in the sidebar).
It seems that having the gradient limited to a small area would make the result more predictable and not negatively impact users with greater amounts of content.