Some random thoughts from working on a proprietary intranet app that manages proposals/jobs/billing and uses some single-page-JS sections:
1. Some users want to have almost an in-browser spreadsheet where a change in one text-cell ripples through dozens of causal relationships. If you choose this, you need to fully commit to all the focus-handling / concurrency / algorithmic issues (e.g. avoiding cycles) that it raises, and sometimes push-back against certain things that just aren't practical.
2. Assuming you don't take the tabular approach, consider task-based interfaces, where there is information about what the user wants to do and you provide an interface that works best for that specific route. (As opposed to "here's a whole bunch of facts, change whatever you want" approach.)
3. Your experienced users will probably want to use tabbing to move between fields for data-entry. You may want to make this an explicit part of the design process, or else they'll get used to whatever the browser default is and then fixing it later will break somebody's learned workflow.
4. Carefully consider what groups of information people will want to enter and then save as a unit, versus pieces where it should be saved on-blur or automatically.
5. If you do use on-blur to save data, make sure you have some feedback so that users know when the field is still in the process of saving. Also consider how to accommodate users who try to make a lot of rapid changes to the same field.