<p>Since the release of Adobe AIR I am wondering why Java Web Start has not gained more attention in the past as to me it seems to be very similar, but web start is available for a much longer time.</p>

<p>Is it mainly because of bad marketing from Sun, or are there more technical concerns other than the need of having the right JVM installed? Do you have bad experiences using Web Start? If yes, which? What are you recommendations when using Web Start for distributing applications?</p>

In my company we used Java Web Start to deploy Eclipse RCP applications.  It was a pain to setup, but it works very well once in place.  So the only recommendation I could make is to start small, to get the hang of it.  Deploying one simple application first.  Trying to deploy a complete product that is already made without experience with JWS gets complicated rather quickly.
Also, learning how to pass arguments to the JWS application was invaluable for debugging.  Setting the Environment variable JAVAWS_VM_ARGS allows setting any arbitrary property to the Java Virtual machine.  In my case:
-Xdebug -Xnoagent -Xrunjdwp:transport=dt_socket,server=y,suspend=y,address=4144
Helpful when you need to check problems during start-up (suspend=y)
I think the main problem for the acceptance of Java Web Start is that it is relatively difficult to setup.  Also, somehow there is this dissonance:  When you have a desktop application, people expects a installer they can double click.  When you have a web application, people expects they can use it right from the browser.  Java Web Start is neither here not there...
It is widely used in intranets, though.