One of the major sources of new users is OU courses. Judging by the trouble they have with the mechanics of iSpot I think we can conclude two things
1) the OU courses fail to effectively teach their students how to use iSpot
2) iSpot's user interface is insufficiently intuitive
New users can agree with observations, but that's not quite the same as making identifications. I doubt that it is practicable to provide observations for them to identify, but if quizzes weren't broken they would be a partial substitute.
It has crossed my mind before that when people sign up to iSpot they should be sent a welcome pack/user guide. To which I now add the provision of "play through" videos showing the correct use of iSpot. Perhaps @miked can record his usage.
The two bits of the old interface that I miss most are Unread (which let me monitor all activity on plants - to do that now I'd have to favourite everything and not miss stuff rolling off the carousel while I'm away from my computer) and the ability to step through carousels. (The first is a functionality issue, and the second a usability one.)