Really the main point was curiosity - Joshua suggested the idea, and I wanted to see if it had legs. I thought it would be a good way to control the rate of growth, fight spam, and limit the number of drive-by users while offsetting some hosting costs. To my surprise, it turned out to be a way to raise Y Combinator levels of funding, with the big bonus of not having to deal with Paul Graham.
I wish I had had the foresight to write down my expectations before starting the experiment.
At least for me--someone unsatisfied with the bloatware del.icio.us has become, willing to try something new, and really not too fussed about a signup fee lower than MeFi's--the cost was actually an attraction. It says "We're serious" and "We want to keep out spammers", both of which are fairly crucial to a bookmarking service.
It also makes the site stand out: probably only URI shorteners are more common than social bookmarking services, and the increasing signup fee differentiates it from all the others (I doubt I'd have given it a second thought otherwise).
