Yi Jiang: It's wonderful to meet other people that also care about presentation. I'll try to respond to your points; I'm sorry that it has taken so long because I missed the separate report on meta rather than the main askubuntu.com site itself.
"Uneven kerning and spacing"
How a particular "title" looks depends on the text in the title, and the configuration and machine that it is being viewed upon. The only way to know what you're definitely seeing is if you can share a screenshot. The Ubuntu Font Family is carefully kerned, with both negative and positive kerning used between many pairs. Currently there ~5,000+ kern-pairs, per weight:
$ for f in /usr/share/fonts/truetype/ubuntu-font-family/Ubuntu-*.ttf ; do\
basename $f | tr \\n \\t ; showttf $f | awk '/npairs/{print$1}' ; done
Ubuntu-L.ttf npairs=4987
Ubuntu-LI.ttf npairs=5957
Ubuntu-R.ttf npairs=5264
Ubuntu-RI.ttf npairs=6055
Ubuntu-M.ttf npairs=5527
Ubuntu-MI.ttf npairs=5697
Ubuntu-B.ttf npairs=6752
Ubuntu-BI.ttf npairs=8366
Having said that, not everything is perfect. Please file a bug report along with a screenshot if you spot something that's still not ideal yet:
The most likely answer though is just that the browsers aren't using kerning by default
"The f and t characters are badly designed"
Again, it's only really possible to know what you're seeing with a screenshot. It could be many things; stylistic familiarity is a possibility in that the 't' and 'f' do not feature full crossbars except in the Ubuntu Mono. The artefacts that you're seeing could also be from synthesis, hinting, or substitution. (For example, it could be a completely different font you're looking at, or an early alpha version of something still in ~/.fonts).
"Bold text is larger than non-bold text"
Yes, bold text is wider. There is a 7% difference in the average width between Regular and Bold:
$ for f in /usr/share/fonts/truetype/ubuntu-font-family/Ubuntu-*.ttf ; do\
basename $f | tr \\n \\t ; showttf $f | awk '/avgWidth/{print$1}' ;\
done | sort -t= -nk2
Ubuntu-LI.ttf avgWidth=562
Ubuntu-RI.ttf avgWidth=584
Ubuntu-L.ttf avgWidth=589
Ubuntu-R.ttf avgWidth=602
Ubuntu-MI.ttf avgWidth=605
Ubuntu-M.ttf avgWidth=614
Ubuntu-BI.ttf avgWidth=634
Ubuntu-B.ttf avgWidth=643
There is also a slight variance in xHeight between the weights (517, 520, 523, 526) although I'm puzzled how 6/1,000s between Regular and Bold is going to be jarring, unless autohinting is in use and one amount gets rounded up, whilst the other gets rounded down. Again, a screenshot would be helpful in trying to understand what you're seeing.
"The Ubuntu font is a display font"
Not really. The previous Ubuntu-Title font used for the Ubuntu logotype was definitely a display font; where-as members of the Ubuntu Font Family have (so-far) been designed for on-screen or print use. It's possible that in the future there might be members of the family targeted for other uses (perhaps an "Ubuntu Thin").
"And isn't suitable for long paragraphs of text."
Have you tried Ubuntu Light; do you find that more suited to your eyes in long-runs of text?