Sewing Success; Sort of

Popping by to show off my latest sewing success!
It's Butterick 5639, in view C.

Yay for pocketses!

I say "sort of" success. And this I say because, well, ultimately I'm lazy with my sewing. I rammed the zipper on the back there, and it looked all good and seamed and flat and lovely.

Then, after a full day of wearing this puppy (at work, no less), my boyfriend helpfully pointed out that, given the right angle, you can totally see through a 2" gap at the base of the zipper. Which happens to coincide with my ass.
So yeah, possible bum flashage aside, this dress was fast (did it in two days! Except for, you know, the ventilation holes at the back...) and is rather flattering.

You can sort of, but not really, see the added bust darts on this pic.

Again, I see that the size I'm supposed to be is actually pretty big on me. The measurements for a size 12 are the same as my actual measurements, so I figured I was safe. I ended up having to: 1) take in the dress 1" at each side! 2) hem it up big time (though, being part hobbit, I do expect that) and 3) create bust darts.

Point 3) was pretty disappointing, as the dress has pretty seams for the bust rather than darts. There was just too much fabric at the armpits, and I'm not sure if that's due to my bust being to big or small for the dress, or maybe my armholes are smaller than the average bear?
Any experienced sewers have any ideas?

I would like very much to make more of these dresses, and would be chuffed if I could determine the ill-fit issues and correct them.

Thanks all!