automatic opt-lambda: As I understand it, the underlying low-level #%plain-lambda is like the original lambda, but in the full scheme language, the automatically available, high-level lambda has support for the functionality of opt-lambda from the old (lib "etc.ss") library. More often than not, that was the only reason I needed to require that library. It's so nice just to be able to reach for opt-args right away, without having to jump through any extra hoops.
keyword integration: Moreover, the high-level lambda also has automatic support for keyword arguments! This means there's a very smooth evolution path for functions: 1) positional when you just need a few arguments; 2) optional when one or two of them have reasonable defaults; and 3) keywords when there are enough arguments with enough defaults that it's too hard for the user to remember their order.
built-in pattern matching: No need to require (lib "plt-match.ss") -- it's already in the full scheme language by default. I stumbled on that by accident!
generalized define-struct: There used to be a huge gap between what you could do with define-struct and what you could do with the very general but hard-to-use make-struct-type. But thanks to the expressivity of keywords, the new define-struct form is much more powerful. Plus it's so much easier to understand what
(define-struct posn (x y) #:transparent)means, as opposed to
(define-struct posn (x y) #f)
built-in list libraries: Many more commonplace list operations are just automatically there when you use the full scheme language, such as foldl, foldr, last, andmap, ormap, build-list, etc. It used to be confusing trying to remember what was in (lib "list.ss") and what was in (lib "list.ss" "srfi" "1"). Now, if I'm not mistaken, I just don't ever need (lib "list.ss") anymore.
my first two columns are mine again: Last but not least, the new #lang module declaration form means I no longer have to indent the entire body of a module.
It looks like the PLT crew have really worked hard on fixing lots of these little inconveniences (some of which I only noticed consciously now that I no longer have to deal with them!).