I know that Pretty URLs are fashionable. CI takes it to a new extreme by unsetting $_GET by default.
Why? Because you should be POSTing or using longer URLs instead. Check out their $this->uri->assoc_to_uri() example on how to "properly" handle hashy query data.
Does anyone seriously think that http://www.example.com/shoes/gender/male/size/18/color/brown is a significantly better URL than http://www.example.com/shoes?gender=male&size=18&color=brown?
Have we thrown out the idea that only one URL should point to a given resource? (I'm betting if you swap around gender and size in the above example it would have the same result and give a 200 response instead of redirecting to a permalink.)
Are GET forms no longer are acceptable for things like searches that don't have side effects? (I'm pretty sure this change completely breaks non-POST forms.)
Don't get me wrong. Pretty URLs are great, and I wouldn't blink an eye if Opinionated Software like RubyOnRails removed GET parameters by default. But in a lightweight framework like CodeIgniter? It seems a bit odd.