There has been some talk in the past about TextMate's undesired tendency to use a software generated bold for the Bitstream Vera Sans Mono font.
One fix seems to be to run:
sudo rm -rf /Library/Caches/com.apple.ATS
Afterwards one will have to reboot the machine, but then the bold shows up as drawn. And what a difference it makes:
Though I don't know for how long this fix will work. And it will still look incorrect when the bold text is light on a dark background.