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It's intentional. For readability.
http://mikeyanderson.com/optimal_characters_per_line
http://www.pearsonified.com/typography/vBulletin font was way too small. You're supposed to be able to easily read stuff when you are say back, with correct posture, etc. Not when you're learned in 2 inches from the screen.
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If the massive font size is intentional for readability, then why is the text in a squished up serif font that is very difficult to read on a non-retina screen? It's too narrow and closely spaced undoes the good that the open line-height does. A nice sans serif with wider characters and greater letter spacing would be better.
I think you need to bring back the resizing thing too. Whatever that guy says about font sizing is his opinion only, and it's wrong. We scan-read text in blocks and process words and passages as familiar shapes, rather than reading the letters every time. Making fonts too large breaks this. What constitutes too large is different for different people.
Also with the extra scrolling necessary, this breaks the idea of 'conversations' that you are trying to achieve, because context gets totally lost as posts get pushed off the screen.
The text resizing thing doesn't seem to work very well in Firefox 31.0. It doesn't use the whole width of the browser window but only a part of its width (unless that's intentional). I'd much prefer it to use the whole width.