My favourite definition of enterprise software comes from John Siracusa:
Enterprise software is when the person buying your software is not the one who has to use it..
I think this is true. I also think there are other interesting axes.
In terms of scale: the cool kids at startups in the valley love to bang on about how their apps scale, specifically how it scales up for system load and user count. And they have to: they charge peanuts on the dollar (if at all) and have to make it up in volume. Enterprise is different: long-term expensive contracts mean that the systems should be – ideally – built to scale for long maintenance periods.
And secondly, who gets to dictate features? Vendor or client? In the consumer “product” world, the vender gets to call all the shots; if the product doesn’t have a feature you want, then the product is not for you.