Size, the dm9 is effectively single tube it's so low profile compared to the previous models.
Weight, Obviously with this major decrease in size, a sizeable weight decrease is another gain.
Detents, the new eyepipe/detents system is more reliable and much simpler with no little magnets to lose of get spun around
Reach trigger, the new reach trigger now not only has all the adjustment of previous ul triggers but now you can adjust the rake too. It's a great upgrade and allows you to completely fine tune the trigger set up to how you like it.
Personally I'd recommend plenty of forward rake similar to the old Ironmen NXL triggers.
Thats off the top of my head, the new 9's are definitely the best marker Dye/Proto has ever produced (except the IM9s).
8-9 is imho the biggest leap the DM has had since 5-6 and well worth the extra cashola if you have it