For basic styling—light padding and only horizontal dividers—add the base class .table to any <table>. It may seem super redundant, but given the widespread use of tables for other plugins like calendars and date pickers, we've opted to isolate our custom table styles.
As for Bootstrap's grid system and nesting grids: I would think all grid systems work this way. I'm not sure how it would work otherwise. It's the same as nesting tables, you'll run into problems if you aren't careful and plan ahead. I would imagine in many ways that nested grid may not actually need to be a grid based on the purpose of Bootstrap's grid system. I bet a series of inline-block elements would serve the same purpose in many cases.
Before-bootstrap is like before-jquery, you really can't understand the colossal contribution of either unless you were there. And just how much of a mess it was.
The dev world is unquestionably better for bootstrap.
Most cases of dislike towards things like Bootstrap is due to not understanding how to use it. It's a case of blaming the tool for not working the way it wasn't designed. Like, as an extreme example, screwdrivers really suck because they can't drive in nails very well.
I believe your first sentence illustrates my point.
That's why there are css resets and projects like this or Normalize. They make base html look the same across browsers before you start creating design-based css.
One criticism: At that font weight, Helvetica Neue looks shitty on Windows machines. Take a look: http://i.imgur.com/peDYi53.png
I've had better luck with open sans at that weight.
With regard to your font weight problem, looks ok to me (Win7 x64 chrome/ff/IE latests):
Good luck with your project!
The first thing that comes to my mind is making themes for this! Should fork and play around.
But how far can be push this ... What about grids ?
I imagine the same problem would exist for flexbox as well, unless the browsers start having proper flex elements.
This isn't really a framework so to speak, it's a css reset. Much like Normalize and Skeleton. There's nothing wrong with it, as more options along those lines are always welcome.