Usability will effect retention and adoption rates - but overall doing a critical business function better or easier than their competitor - Quickbooks - will be the winning factor here. This means financial statements, tax planning and integration, and proactive alerting and real-time status. Branching out to integration with Freshbooks for invoicing and billing (as I believe they have done) along with multi-user, tiered accounting and HR integration (they do most of the accounting for small businesses), and resource planning.