Of course, even in the on-premise world, vendors typically need to show their product direction as well. It's doubly important with SaaS solutions.
An analogy from the publishing world illustrates the differences. An on-premise application is like a book. Whatever words and pictures are bound between the covers as you walk out of Borders or open the package from Amazon is what you've bought.
A SaaS solution, by contrast, is more like a magazine or newspaper subscription. As a customer, you're not exactly sure what stories will be covered next week or next month, but you're paying for the publication based on the expectation that it will provide you with valuable content over the life of your subscription.
Any lessons that we can borrow from the publishing world that can help us?
Some publications prepare an editorial calendar. They identify the general areas they'll be covering over the coming year. They usually don't provide much detail - a paragraph or two - but enough to convey where they'll be focusing their editorial resources. SaaS vendors can do something similar. Show the prospective customer the general direction of your solution, and given them a sense of where you're applying your development resources. There's no need to provide many details, especially as you get further out on the calendar. In fact, in an agile development environment, you're not likely to have many details far in advance in any case.
A second idea borrowed from publishers is to show your back issues. Let the prospective customer see that you have a history of delivering valuable content quarter after quarter. Show the timeline of product enhancements over the last few years. Establish a pattern.
One final thought on this topic for now. Don't obsess over the potential hazards of disclosing your future plans. Some of that deeply-seated caution is a remnant of the on-premise world. When companies introduce products every 18 months, if you could bring out a differentiating new feature, you could truly steal a march on your competitors. In the SaaS world, competitors should be able to respond much more quickly.
Moreover, the whole notion of springing a surprise usually doesn't sit well with your large enterprise customers. Think about it. Before the latest enhancements get rolled out to hundreds or thousands of people in their organization, the folks who are responsible need to carefully understand it and prepare for it. They'll expect advance notice, not a surprise.
For people like me who have spent lots of time in the traditional on-premise world before the SaaS world existed, I understand that this reticence about disclosure is a tough habit to break. NDAs and embargoes were all standard fare. Of course, in some cases they're still required. But it might be better to leave much of that behind.