In reply to what makes it text vs display?:
I think I' going to regret this...
Your mathematical/mechanical comparisons are the wrong tree. Take a look at Trump Medieval. A good text font. Now look at Adobe Garamond, another good text font. Sorta knocks the wind out of the cap heigth/ascender heigth/x-heigth/descender depth/small cap height as a matter of ratio, doesn't it?
The one think you can say about text type is it has to be comfortable for a 300-page book.
& my current rant, which is not enough attention is paid to how the type appears. Printed offset, on a certain kind of paper -- coated versus uncoated. Or printed on a laser printer on an 8.5 x 11 sheet of office paper. Or as pixels on a screen. And, uh, which screen?
Whitman, printed offset on an uncoated sheet, is a bit light. Kent knows this. It is worse on a coated stock. It is beautiful printed with toner, as with a laser printer.
Never used Parkinson. Doesn't look very good to me at
http://fontzone.net/font-details/Parkinson-Medium/
but that's not a fair way to observe the font. I think some of the letterforms suspect, but in the final analysis, it's the use that counts.
It will probably print like Merlo, I suspect. Merlo doesn't print well offset, on an uncoated sheet set larger than about 10-point (& that's an iffy combination), but is wonderful on a coated paper where the ink doesn't spread much.
http://www.felicianotypefoundry.com/cms/fonts/merlo
The thing about type is, it hasn't much value until used. So look at that.