"Same old Software, as a Service (SoSaaS) — Take any old software package, run it up on a server in a data center, do a bit of financial engineering so customers can pay on a monthly plan, and hey presto! you've got an on-demand application. To be fair to Microsoft, there is a version of Exchange Server that's specifically designed for high-volume hosting providers, which makes it a little bit more sophisticated than the efforts of most conventional software vendors. But the fact remains that this is a software package that is designed and built for on-premises installation, and therefore it always will be less than ideally suited for operation in an on-demand environment (as George Ou points out for example, it passes attachments around instead of sharing a single master copy, thus ensuring abysmal performance on anything except a high-speed local LAN connection. How dumb is that?)
Software that actually works — The on-demand model isn't about delivering software per se. It's about delivering the results of successfully using the software. When email is delivered as an on-demand service, what you get isn't a hosted email server — the internal mechanics aren't of any interest at all — what you get is a commitment from your service provider to make email work for you.