That's because that's a jump from 94 DPI to 101 DPI, rather than to 109 DPI. You should have noticed a difference but it wouldn't have been as pronounced as a 27" display.
Because that scaling is pretty nasty. With Windows XP style disabled it's certainly better but a lot of applications will appear blurry - I couldn't put up with it for more than a few minutes. Whereas vector art scales without issue.
This is up to the developer to fix. You don't need vector art which is computationally expensive. If the program developer followed Microsoft guidelines the program would look good. They suggest to have bitmaps for 100%, 125%, 150%, and 200% which are then shrunk to fit sizes in between. Even if vector art was supported, I don't see why the developers who have still not fixed their programs will suddenly update them to support vector art
Programs that are designed to be DPI aware look great. The programs which aren't DPI-aware and when using Vista style scaling will look blurry because you're not increasing them in an integer multiple. There is nothing the OS can do to fix this issue. Take any screenshot with text and put it in an image editor and increase the size by anything other than integer multiples and it will look bad. If you have a really high resolution screen you can set it to 200% and it would look the same as when Mac OS pixel doubles older programs which aren't DPI-aware.
Here is a comparison between 100%
. I think the 125% looks great. The only issue I notice is the Microsoft Word icon in the taskbar. But this is Office 2007, Office 2013 doesn't have this issue and I don't think 2010 does.
Edit: And here is a photo of Windows 8 at 125% on my laptop
. Office 2013's icon looks fine. Only thing wrong with this screenshot is the Skype icon and the TPFanControl system tray icon. Skype really needs to fix that, but at least the program itself is DPI-aware.