First thing I want to say is that I think that Paragon leveling added a nice touch to D3. On the other hand, I think that there is a nice amount of room for improvement.
Each character has to Paragon level which means you generally just choose one. I love my Barbarian and he is going to be my Paragon toon, but sometimes it would be nice to switch it up a bit and not feel like you are being penalized for it.
I have a level 60 DH and am also working on my Mage. I cannot really play them at max level because it makes me feel guilty since I should be working on my Paragon level. Well guilty isnt really the word. More annoyed.
I think that there is a simple solution to this. Account bound Paragon levels. As you hit 60 on other toons, the Paragon levels match up and gain XP on the account, not just the character. That way if you feel like playing a different toon, you can without thinking about the paragon level xp you are missing out on with your main.
I would probably sample a bit of every class at max level if this were the case. I really hope Blizzard thinks about this sometime soon.