The only thing stopping XSLT from gaining traction, ironically, is the web standards community. If we can get widespread buy in, people will develop XSLT 2.0. Empower user experience designers to be able to have complete control over their markup by using XSLT. In my mind, Symphony is the tool of choice for this. I've been using it for HTML/CSS prototypes for the past five years. I'd never use anything else, if I had the choice.
The only reason CSS is as popular as it is now is because there are evangelists like Zeldman, Shea, Bowman, Cedarholm, Clarke, etc. Inexplicably, Zeldman and company have not mentioned XSLT on A List Apart since 2002: Using XML. When I asked the Web Directions panel why we rely on proprietary APIs instead of open standards such as XSLT, Simon Willison and Jeremy Keith were oddly hostile to the idea of XSLT. (Search for XSLT in the transcript of the Web Directions panel discussion on August 20th, 2010.) So, that night, I wrote a rebuttal to their argument.
I would challenge Jeffrey Zeldman to champion the topic of XSLT and write the next best seller: XSLT for Designers. I would write that book, if there were no other takers.