Scrolling can at times be very annoying now in Mac OS X -- something that simply didn't used to be the case (no one complained about scrolling before). But with these new scrollbars the following things can happen:
1. It's very hard to navigate a long list. For example, if you are in Mail.app somewhere in the middle of your messages, swipe scrolling all the way to the top can get very annoying, very fast. On iOS this isn't a problem because you can tap the top of the screen to zip up to the top, but this doesn't exist on OS X. Sure, you can always grab the knob once it shows up, but...
2. The knob is so small now that it is actually hard to grab. So when you do need to grab it, it sometimes takes a few clicks to get it. I feel bad for anyone with bad eyesight.
3. The scrollbars often overlap the content in incredibly annoying ways, for example if you are trying to select the last item in a horizontally scrolling list, the horizontal scroller will sit right on top of it and you have to wait for it to go away or you end up clicking it instead: http://i.imgur.com/NDxTb.png . This happens all the time with the Open panel.
4. Not to mention the fact that it isn't immediately obvious anymore when you have additional content to scroll.
point 2: Agree, but I think on balance in most situations, I'd rather have this than the old system. I have a feeling this can and will be refined somehow.
point 3: Agree, but there were annoyances with the old scrollbars, too, although somewhat less obvious ones.
point 4: It often is still obvious from context; for instance, alphabetized lists whose visible contents end in items beginning with "b" are a hint. :)
Zero?
Edit: For some reason I'd read the parent as saying that you didn't need visual feedback on a touch screen, which of course, you still don't have with a trackpad. Not sure how I got that.
And, to be honest, touch gestures in general are a very welcomed change to the OS. I find the use of a mouse only needed for precision control (say, graphical work or gaming!)
Probably around 90.
b) Apple didn't eradicate anything; you can still turn the old-fashioned scrollbars back on if you choose. They just changed the default to suit what Apple believes to be what most users will choose, now and in the near future, with the hardware that most of them will be using most of the time.