Not very much on the language itself, but a few things will:
get_pos(), you'll be able to simply write
position. Same for other things that were "properties" on nodes but that needed
set_ to be read and changed.
_input etc will be called by default if they are present in the script, no need to explicitely
set_process(true) etc anymore.
Dictionaries will be sorted by key insertion order when using a
for loop to iterate over them. So I guess if you load some JSON you will get keys in the same order as encountered in the JSON file. While I don't like this (due to performance hit even when you don't need it) it can come handy in several cases.
A more efficient syntax to iterate between 0 and N:
for i in 10: # same as for i in range(0, 10)
range was creating an array behind the scenes, the new syntax does it without.
Also, a lot of classes and function names will change (but that goes beyond GDScript). For a more detailed list of compatibility breakage, you can have a look at this doc https://docs.google.com/spreadsheets/d/1SqLGKpF5B5KzYnY2JzuuP71tsR8WeXZn1imMvHkoKDc/edit#gid=0