My opinion is that the site will only retain new users if the interface is user-friendly and they are engaged by a community of welcoming, encouraging and helpful people.
I think most new arrivals' expectations would be that this is a wisdom-of-crowds kind of place, where they will post an observation of something that has sparked their interest and curiosity, and will be engaged, enlightened and appreciated by others. I imagine it must be disheartening and discouraging to come here, take the time and trouble to post your obscure ichneumon or unidentifiable tunicates or Ivy Bee, and get no response whatsoever. Why would you return?
There are experts here, of course, but they are as busy as the rest of us (no more so) and, with some notable exceptions such as lavateraguy and markwilson, can be rather aloof and curt, even dismissive. If something matters to you, you make the time; if it doesn't, you don't. Some experts will be interested in helping others, and others won't care at all.
What I've enjoyed on iSpot, besides the access to records to assist my study of Trichoptera and Plecoptera, is being engaged by some of the super-users, such as dejayM and HB, who have coaxed and mentored me, helped me to make identifications more meaningful to others, rather than just saying it's species X. As corny as it may sound, I do feel part of a community.