I last sailed in October 2013.
Some of the things that have changed that I've kept track of:
Tipping is automatically charged to your on board account (changed happened April 2012 I believe so you may or may not be aware);
Princess Gathering, Disney Junior and Anna and Elsa are now ticketed events;
Palo is $30 for brunch and dinner;
A Shwarma place was added to the Magic;
New waterslides (Aquadunk and a yellow twisty slide) were added to the Magic;
Avengers area added to the kids club on the Magic (actually, the whole are was re-imagined) WDWInfo pictures
http://photopost.wdwinfo.com/showgallery.php/cat/1624
Carioca's replaced Parrot Cay on the Magic (new look and new menu);
The buffet was redone and renamed to Cabana's (style is now similar to Dream and Fantasy);
Adult areas were redone;
Tangled the Musical will be starting in November 2015 aboard the Magic
Changes to
Castaway Cay - new Cabana's (opened early 2012), summertime freeze stand opened recently;
New health questionnaire;
Bibbidi Bobbidi Boutique has very recently been added featuring Anna & Elsa’s Boutique
Some small menu changes to the main dining room menus over the past couple of years;
The beds no longer split and are now a queen bed with a wooden frame;
Prices for basically everything onboard or at Castaway Cay have gone up.
That's about all I can think of.