The new design is great, but the font is really killing me. Here's why:

###Uneven kerning and spacing

Have a look at this title. The space between n and e in uneven, r and n in kerning, a and c in spacing are all wrong! There are a lot of other examples, but a font like this just isn't conducive to reading. If you want more, have a look at the headers for each category of applications in this question.

See also: http://ilovetypography.com/2007/09/15/type-torture-kerning/

###The f and t characters are badly designed

Words with t and f in them look really weird. For instance, Firefox looks like it might actually be two words when bold. The main problem is with the way the ascenders are designed. Suffice to say it'll only look good on headers

###Bold text is larger than non-bold text

Really, it makes any string of text that is bold look out of line. The x-height difference is jarring enough, but the letter spacing is even worse.

###The Ubuntu font is a display font

And isn't suitable for long paragraphs of text.

#Proposed solution

is simple. This font can be used everywhere - even at small sizes, small doses can still look good. For example, the user info box that appears with every post. However, for large block of text, (I'm thinking stuff in .post-text mainly), can we please stick to something more conventional? The Bitstream Vera family is a good choice, as is the Liberation and the Nimbus Sans family. You'd want to keep the usual Arial/Helvetica last, since it's Ubuntu we're talking about here.