Time limits put people off because you can't run a small project on the service, so any time you invest in learning it is going to be wasted if you can't/won't pay later on.

I actually really like IBM's Cloudant model where they change incrementally for everything but don't bill you if your usage for a month is under $50. That way I can happily build a small side project knowing it'll only cost me if it gets moderately big.