(If you watched the
O'Reilly live webcast, then give this a try:
http://cemerick.com/2012/05/02/starting-clojure/)
Riffing on what technomancy said nearby, a lot of the issues that Clojure addresses are so taken for granted in most of the broader software development world, that it can be challenging to wrap up its benefits into a tasty, easily digestable nugget comparable to "Java gets rid of malloc and pointers" (sorta). Some have taken some whacks at doing just this, including Paul DeGrandis' talk you linked to already, as well as "Neal Ford's master plan for Clojure enterprise mindshare domination": http://blip.tv/clojure/neal-ford-neal-s-master-plan-for-cloj...
That said, the benefits of immutability and pure functions are generally obvious to any reasonably-capable programmer (my personal bias shone forth) given 10 minutes in front of a whiteboard. Can we get the, say, 10 million Java developers to sit still long enough for that? No, probably not. Maybe a tenth of them eventually, maybe. And, if a bunch of them head over to Scala because they like the look of {{{}}} instead of ((())), so be it.
A more interesting question to me lately is, what is the objective of language evangelism? Let's set aside any notion that Clojure will be "the next Java", or even achieve the same penetration as Python or Ruby. For those of us that use the language, none of those goals are particularly important. "How many developers use language X" used to be a critical question when the language you used largely delimited the reach of your eventual application/system/whatever; that is hardly the case anymore. I can write my server side in literally anything, and transpiling to JavaScript or C for client side as necessary is either within reach, or a solved problem depending on your preferred language.
So, what should be the objective? Having a large enough pool of talent to meet business objectives is the only thing I can think of that is an actual potential blocker. I know lots of people that know Clojure that would like to work in Clojure, and there are more people like that every day. Seems like the vector is trending positive; we only need to make sure it stays that way.
(This leaves out the enterprise guys to a certain degree, but that's largely due only to their [and their managers'] self-inflicted risk profile.)
Finally, I leave you with this:
http://www.indeed.com/jobanalytics/jobtrends?q=clojure%2Chas...
Note that those are absolute trendlines, not growth. You can very reasonably say that Clojure is the most in-demand functional programming language. (…for some definition of 'functional programming language' ;-)