We see a lot of both sides of this at BR (http://buildingrobotics.com/) -- we are actually BACnet experts, but also provide a really novel user experience on top of this legacy protocol. One thing I'd add is that many of the "legacy" industries have evolved in a way that's very different from what the Internet crowd expects and so you can't always make the same assumptions.

Also, it might be "unsexy" but buildings consume 70-ish percent of US electricity, a lot of which is controlled by BACnet and friends... and it turns out there's a ton of low-hanging fruit that's only a (carefully crafted, ASN.1-ish) UDP packet away..