I have read numerous times that EJBs are heavyweight...yet last night I was going through the Java EE 6 tutorials on EJBs and they seem to be just regular Java objects, except they can have annotations like Stateless or Singletons. Other than that the functionality they contained in the (admittedly very basic) tutorials was very standard, just like a model class in an MVC application.
There are a few causes of EJB beans being heavyweight.
The first one is Rod Johnson's rhetoric. For people to adopt Spring it was important for Rod to spread the idea that EJB is heavyweight (and Spring thus automatically lightweight). Springsource is a commercial organization and Rod a clever businessman. The more people use Spring, the bigger the percentage of them that will eventually buy a support contract from SpringSource.
The second source are Spring zealots who never tried EJB, but always heard from Rod and other zealots that EJB is heavyweight. They are simply parotting the song they've heard.
The third source are people burned by EJB 2 in 2004 and before. This version of EJB was indeed heavyweight. People turned their back to EJB back then, never looked back and are completely oblivious that EJB 3 happened, which is as you have observed a very lightweight and easy to use technology.