Few things I noticed off the bat (class-specific stuff is compared to the official calculator):
When a skill, rune, or passive is filled in the official calculator, it moves you to the next empty slot. Ours doesn't, but it would be a nice feature to implement.
The class images look awesome, but they're big enough that once the calculator pops up, I have to scroll down to use it. Any way those could shrink a bit once a class is selected or be moved to the side?
The runes and passives UIs have a lot of extra room, but still require you to scroll over the icon to see what it does. It's a much easier point of comparison, IMO, to be able to see the effects of everything at once and make your decision from there. The way ours is set up right now is kind of like playing Memory.
Classes are arranged alphabetically, except the Demon Hunter is last for some reason.
Clicking on the rune slot when a skill is empty just tells you that you need to "Select a skill for this slot first." Why not have it open up the skill list if a skill isn't selected yet?
The text on a lot of skills runs together a bit and makes it harder to read. Different text colors and/or extra linebreaks between things like skill costs and descriptions would be very helpful, I think.
I know you're aware of the problems here, so I won't mention them.
Tempest Rush uses the Dashing Strike icon.
Passives Near Death Experience and Guiding Light use the wrong icons.
The Arcane Dynamo passive has the wrong icon.
Passives Fierce Loyalty and Blood Ritual use the wrong icons.
It would be handy if when I'm viewing the calculator for a specific class, the icons for selecting my class were moved to a vertical menu on the right or left of the slim calculator or were minified. It's semi-annoying having to scroll down after I click on a class.