Tagging, Tagging, Tagging...this means that you can have articles 'link' to multiple 'tag' areas at the same time. This can create an effect of having an article exist in multiple categories.

The back end of 3.1 is completely workable on a small mobile device. Just the other day, I was able to successfully change administrator settings while in the car on an samsung galaxy, which is unheard of in older versions of joomla

Yes, you have some getting used to in the 3.1 interface, but I feel it is faster to work with, and the displays are faster as well

When selecting compontents for your website, it is also a good idea to stick with tools that work for Joomal 3.x, if they do, it means the developer is 'seriously updating their tools', and old 'bad code', just won't work on the newer versions of joomla.

What about 3.5??? From what I have heard the next 'long term release' will be out in the Fall of 2014, so you have some time, no matter what release you choose.