The valuation metrics for deals such as Microsoft's $100 million acquisition of Sunrise Calendar have raised plenty of eyebrows. The app-maker was "unencumbered by revenue", didn't seem to have significant barriers to entry, and only had 12 team members. However, valuations on this type of transaction are often better expressed as a multiple of invested capital rather than revenue - because in most cases there is a lot of capital in play, and the buyer believes that it was invested in something valuable.
When deals like this happen, drivers I would look at a) robust IP portfolio protecting critical ground in an emerging market, b) high-value software that gives the buyer a time-to-market advantage in a high-value market, c) rock star team (more important than most people realize).
A large, fast-scaling user base can be important but not as much in hit-driven segments (games, lightweight apps) where the fans are fickle and monetization is a challenge.