You can really use all keys, even 6# and 6b. As far as I know, only Equal and Lehman-Bach temperaments are suitable for this, because they don't contain any Pythagorean thirds in any key and have all fifths considerably wide.
Every key is different, has other color. This was widely used in Baroque, different keys were used for different purposes. Concept of multiple different sounding keys is completely lost in Equal temperament – there is in fact only one key and every other is just transposition of it.
Like some other historic temperaments, it uses different sized intervals, so there is not just one major third, but more of them. So the third C – E will sound different from D – F#. This makes music more vivid. In contrast, in Equal temperament are all intervals the same.
Because it is based on whole number ratios, chords are more clear than in any other tuning usable for all keys. There is just less harsh, less beating in chords on the average.