My friend is into music theory and programming and is working on a chord progression editor: http://evanshort.name/chords/. He's always looking for constructive feedback to make it better.

I was expecting to be able to hear the chords played in sequence according to the tempo, so I was confused there was no "play" button. Gradually I realised the "tempo" just controls the arpeggio rate (one 4/4 bar of 16ths), and there is no way to string chords together and just hit "play" to set them all going.

I haven't tested out the chord spelling issues MiskyWilkshake brought up, but this is a significant (if tricky) issue.
Firstly, enharmonics must relate to key. Abm would be correct if the key was Cb or Gb; G#m would be correct if the key was E, B , F# or C#. Naturally this gets problematic for chromatic or borrowed chords, and in some minor keys, where you have both sharps and flats. I can imagine that this too tough a nut to crack for a program like this, because of all the variables and the rules governing each one - and because a lot of the likely users wouldn't care anyway. (It's folk like us who care!)

/r/musictheory Thread