Probably the most important change is the one of the user interface including the addition of the entirely new Arcade game menu, a new streaming launcher, QuickNav and graphical improvements. There are also new editor modules. But yes, the UI changes are the most noticeable and you will probably need some time to get used with the new look of Starcraft II (in terms of UI, of course).
Here’s more about the new Arcade button, straight from Blizzard: “The StarCraft II Arcade has arrived! To make way for this new feature, significant changes have been made to the StarCraft II user interface. Two new buttons have been added: the Arcade button, which will act as a portal to a brand new custom games experience that features dramatic improvements in finding, rating and playing custom games, and the StarCraft II button, which will provide access to classic single player and multiplayer experiences, including the StarCraft II campaign, multiplayer ladder, melee maps and more.”
But there is a whole lot more in terms of changes, so while you download the Starcraft II patch 1.5.0 to update your game, make sure you check out the full change list over at Battle.net.