http://stackoverflow.com/questions/new/c%2b%2b?show=all&sort=recentlyactive&mode=any
compared to
http://stackoverflow.com/questions/tagged/c%2b%2b
the first URL has a pile of noise in it. The second is short and simple.
If you try to shorten the first url and type in:
http://stackoverflow.com/questions/new/c%2b%2b
the page auto-refreshes with the extra noise at the end (presumably based off what your last settings where). The automatic addition of the noise hides where the user intended to go, and instead shows the user where they are.
The noise is "needed" or makes sense when the user changes their settings. But if when the user doesn't change their settings, SO rebounces you to "what if you picked your current settings URL", which makes the URL less human readable.
A practical problem is for people who use URL history as a form of navigation. Long, computer-generated, option-encrusted URLs are not useful: short, actual URLs a human can type and read, are useful.
This also makes manually editing the tag you are looking at really annoying. Prior to the new nav, you can alt-d to enter the address bar, backspace over the identifier, type in a new tag, and press enter. Doing this now is no longer practical, because a pile of auto-generated noise is added after the tag.
This last problem is bad enough that I'm turning off the new nav. I am a programmer, I use the keyboard over the mouse, and this has damaged keyboard-based navigation for me.
http://stackoverflow.com/questions/new/c%2b%2b?http://stackoverflow.com/questions/new/c%2b%2b?show=all&sort=recentlyactive&mode=anyor whatever I was "last on". The arguments after the?are seemingly only needed if I want to change my settings: settings are preserved without that noise.