I was referring to the 2.7.3 docs. Personally I don't like the brighter 3+ docs.

This is much more readable to me despite just the sidebar being different: http://docs.python.org/2.7/library/io.html

I'm really curious as to what other subtle changes contribute to that but I just prefer the 2.7 docs over 3 given that the text area looks kind of similar if you ignore the sidebar.

It's the sidebar. At least that's what makes the difference for me.

The font and color of the 3.x sidebar are so similar to the main text that I find my eyes wandering into the sidebar as I read the text.

In the 2.7 version, the dark blue-green sidebar with inverse text is very different looking from the main area. It makes a hard stop on the left, so my eyes don't wander there as I read the main copy.

Testing this theory, if I collapse the sidebar in both versions, my discomfort with the 3.x docs goes away. Now they seem pretty similar - a few things I like better in the new, a few were better in the old, but nothing major like the sidebar.

I do like having different sidebar colors for the two versions, so there's a constant visual cue as to which version I'm looking at.

Another experiment: I viewed the 3.3 docs in IE8 so it knocked out the rounded corners. I definitely like the square corners better. A particularly bad example is the top of the sidebar margin after you collapse the sidebar - it looks really weird with the round corners. Much better looking in IE8 with the square corners.

I also get a much better sense of the top navigation in the 2.7 version. In the 3.3 docs there's some little stuff above the first heading, but it doesn't really feel like this is the top of the page. If I've scrolled down a ways and then scroll back to the top, I get the feeling that I might as well just keep on scrolling past the top. There just isn't a sense of it being something different from the rest of the page content.

(Yes, I know how to use the Home key - I'm not talking about physical navigation but how the page feels.)

In the 2.7 version, there's a clear stop at the top - and at the bottom - just like there is at the left. Also, all three margins connect, so it really sets off the main text. Everything looks pretty much the same in the 3.3 version; it doesn't have this clear separation of navigation from content.

You've hit the nail on the head. This describes perfectly the annoyances I come across while going through 3.3 docs.