The newer ships have more dining venues and are obviously more modern in decor. Anytime you have a somewhat larger ship you are less like to run into service glitches.
For example - on a ship with 200 passengers you might find the room service staff is asleep at 2:00 a.m. - or ta least understaffed, but on a bigger ship they know there is a bigger chance of getting a call.
You will find a bigger spa with more offerings, a larger swimming pool and hot tubs, etc. But the biggest difference is the variety of dining options.