katfang did this evaluation for me (hasn't taken the UI class). I showed her Nielsen's 10 principles and to try using every feature, that it was a prototype and not everything was implemented yet. Afterwards I had her rank problems Cosmetic, Major, Minor, Catastrophic. Since she didn't really know UI principles, I went through and tried to match problems with design principles afterwards.

Minor. I got confused by the preloaded songs and just wanted to record something.
-> I want the focus to be more about making a new song than browsing the old recordings. Move the "new song" button more front and center.
learnability

Major. Your long notes don't seem to actually sustain during playback
bug

Minor. I didn't get to name my song when I saved it. :( I don't know what the name of my song is so I don't know where it went. :( I guess it's probably Untitled 16.
safety, visibility

Major. It's not obvious that by clicking on the name of the song I can go back because everything else has a button and it takes up an entire row. An "Edit" button would probably work (next to rename, save as, export, delete)
visibility

Minor. I don't think export works.
bug

Minor. Might be nice to be able to zoom in "vertically" rather than horizontally. Since my bottom line was so low, it squished my top lines and I wanted to be able to focus on my top lines. This might be more imperative if you have a large range of notes.
visibility, error correction.

Major. I can't zoom into my notes, so it's almost impossible to select them to edit them.
visibility, error correction.

Minor. Could not figure out how to edit notes. When trying to edit:
I tried to drag lines up and down and tried to drag the end points left and right, but nothing moved. [This is not in edit mode, but where you can see all the notes]. Finally came upon edit note while trying desperately to zoom
visibility, consistency

Major. On note edit screen: tried to tap up and down to make note change but after several tries figured out that swiping is the only thing that works
visibility, consistency

Cosmetic. When editing my note, I can see my note in the background, but it's not all like centered and stuff.
consistency

?? my note disappeared. I might have recorded over it. 

Minor. I can't move a note past another note. I'm not sure if this is actually desired since you probably do only want one track to have one tune, but it'd be nice to make my note move entirely past another note.
consistency? visibility?

Minor. Periodically, I can't hear my note when trying to edit it.
bug

Major. I wanna put back a note that I deleted!
user control, error recovery

?? I can't seem to move the cursor consistently.
I was trying to drag the cursor to move it, finally realized you tap to move it. Didn't realize although I accidentally moved it all over the place when trying to tap notes
visibility, intuition

Cosmetic. deleted song seems like it should slide up instead of sideways.
consistency

Minor. I can't zoom out anymore. :(
Apparently when the song loads, notes start small, then when you zoom song width snap to screen width and then zoom seems to not work anymore.
visibility

Cosmetic. I changed tracks from red to green and most of the red notes went dark, but two of them didn't.
bug

Catastrophic. BUG: I can't hear my starting notes for some reason. This seems to be tied with the thing above. The notes that were still bright after I changed tracks were the ones I couldn't hear. 
bug

Minor. Was able to edit green note while on red track.
Not sure if good or bad. Doesn't seem consistent with "on red track -> can only record red track"
consistency

Minor. I'm not sure what other programs have, but the whole leave and "Are you sure?" when you have unsaved changes ... I feel like "I want to save" should be the first option. I clicked that the first time I saw it and then was confused when I didn't see my music in the list.
consistency, error prevention

Major. Had a little bit of trouble pressing the rename button. It was too skinny.
Fitt's law?

Major. Doing "Save as" didn't give me a visual cue that it was saving another file 'cause it just slide the old one down, and made me rename the top one, which was no different (visually) from renaming a file. 
visibility

Cosmetic. I don't understand why the search bar appears at the bottom of the screen.
consistency

Things you got right:
-> alert on trying to leave when song wasn't saved
-> having multiple tracks
-> scroll gesture works as expected