I'd like to see Jackson used instead of FlexJson in Roo, and only used when annotated with @RooJson. It is already used as the default json view in Spring MVC (MappingJacksonJsonView) and powers @RequestBody and @ResponseBody (which are very handy).
ROO-1302, Stefan said that a reason for the choice of FlexJson was its handling of cyclic references. Would the new @JsonBackReference and @JsonManagedReference annotations in 1.6 help? (http://wiki.fasterxml.com/JacksonFeatureBiDirReferences)
He also mentioned an include/exclude syntax but I would argue that if you don't want more annotations in your domain you can use views but that it seems to be a logical place for them (and in keeping with the rest of Roo?).
As a side note, I've been using Roo for my last few projects and very much enjoying it. Thankyou =)