I don't like the new site.
Just yesterday they fixed one thing that bothered me about it, which is that anytime you highlight, you would get a fade-in pop-up on the side of the text about journaling or highlighting it. This was pretty irritating since I am in the habit of highlighting text while I read. Now there is a constant black bar that tells me to sign in to journal and/or highlight, which is also annoying, but less so.
Secondly, the new layout is not fluid horizontally -- it is horizontally centered and seems to be tailored for 1024x768 audiences. It's much worse attempting to read scriptures or articles on my 1920x1080 screen with gobs of excess space on the screen, now only occupied by blue gradient. The text used to flow well and take up all available screen space due to the simple layout of the site -- the revision not only horizontally centers everything, but even cuts the container down, with "Share" and "Download" links on the sidebar. The text is spaced awkwardly and it's just a great big waste of screen real estate. I read the print version most of the time (now I use the old one since they brought the old scriptures back), but it doesn't have footnote toggle and the spacing is still weird; at least, however, it doesn't waste 50% of my horizontal screen space. Loading footnotes can be tiresome, as the site makes an AJAX call on load; I'd much prefer preloaded footnotes. The new chapter selection page makes it harder to find the chapter you want -- you can either scroll a lot and use the same number of clicks or click on the jump menu and add an extra click in there.
There are some cool things about the new site, and I like the use of HTML5 font faces and other things like that, but for the most part it's less usable for more shininess. Compare http://scriptures.lds.org and http://classic.scriptures.lds.org and tell me which you'd rather read from.
Clearly the first one (ignoring for now that scrolling on Safari was jerky when I tried). The old one is fluid which is just a bad idea for long texts. Much more than about 100 characters per line reduce readability, using all the space on a 1920 pixel wide monitor is just a bad idea.
The new version could maybe have about one third wider lines but the seventy or so characters are a good number for texts on the web. The new version is beautifully typeset and has about the right line length.
Using all the available horizontal space on the web for text is nearly always wrong.
The guideline is to have about 100 or so characters on one line.
All that being said, I do find that raw text that spans the screen to be harder to read...mostly because it's easier to get lost ending a line on the far right of my screen and returning to the far left (especially if the font it small, something that zooming the page up doesn't seem to have as much problem with).
I think a better middle ground are n-column layouts, like in newspapers, or most holy books. http://randysimons.nl/125,english/129,multi-column-text/