"Ability to pull and reference data dynamically from online sources. For example, imagine a spreadsheet cell that pulled the current stock price of GOOG every time it was viewed. And the rest of the spreadsheet would naturally update automatically."

Yes. This feature implemented well would create a really cool app. What would be even more interesting would a situation where several different sheets could pull data from each using a clever protocol to avoid the churning of values. Imagine different enterprises which each had online sheets describing their current production abilities and current supply needs. With a clever protocol, their production processes be semi-automatically coordinated (and remember, semi-automatic, some throttling is necessary to prevent self-referential cells from feeding back in an unhelpful way).