I'm going to hijack this thread to do a comparison of the two styles on a question page. As it happens, some of your impressions are not validated by a direct comparison — in particular, I don't see any way in which the main text is lighter (maybe different platforms used or now use different fonts?).
I happened to have a tab open from a few days ago, so I compared the old (rev 2015.3.13.2386, CSS f6eec6262290) and new (rev 2015.3.17.2391, CSS fd3ea6a99fc5) appearances (Chrome 40 on Linux). I haven't tried to read the CSS, I'm just going by visual comparison.
The name of the current section at the top is now in bold, which is a convenient discreet clue to navigation. The title bar is on a darker light blue background for some reason.
The question title is now a a lot better contrasted, but no longer bold. This makes the title less wide, so easier to read at a glance. Because of the better contrast (significantly darker blue on white), the title is about as emphasized as it used to be, and it's easier to read.
The new main text font is very slightly larger. As always this makes it slightly easier to read at the cost of cramming less text per square inch.
The vote buttons are a little larger, and the score is less bold. I didn't need the larger vote buttons on the desktop website (whereas I really wish the vote buttons were larger in the mobile app: I keep missing them on my phone). The favorite button now looks inert, it was better before when it looked like something you could click.
The tags are larger. Why?
“Add a comment” has the link color. It used to have the secondary button color. Since this is not a link that opens another web page, secondary button is what it should be. (Or else all secondary buttons — “share”, “edit”, …, should use the same color as links.)
The buttons above the edit box are now markedly larger. I don't see the point (but then again I never use them). They're now in black and white, I think the old colors made them more distinctive. In particular the help button could stand with being something more visible than the new washed out gray. I still can't figure out what some of the buttons do without looking at the tooltip.
The text in the edit box is now markedly larger. Why? I was happy with the old font size.
The larger font combined with a slightly narrower box makes less text fit in the answer preview.
“Asked <date>” and “viewed N times” on the right are in a different font. They're less emphasized, which suits secondary information.
The related links on the right are now in link color. That's better.
There are a few tweaks in the small type at the bottom and to the meta box. I don't care what happens to the bits no one pays attention to.
- Top sections: nice new feature, but the old background was better
- Title: significant gain, no real cost
- Main text: neutral
- Vote/favorite buttons: slight regression
- Tags: why are they larger?
- Add comment link: slight regression
- Edit buttons: slight regression
- Edit box: larger font, for better or for worse
- Answer preview: slight regression
- Asked/viewed numbers: improvement
- Related links: improvement