Patch notes are a good place to start. I’m not sure what platform you play on, but the changes are pretty consistent from platform to platform:
The long story short is: they’ve made excellent balancing changes, added great new content and fixed a bunch of bugs. Opinions vary, but I am among the extremely satisfied.
And, welcome back! You picked a great time to return: some great New Content is just around the corner, and it won’t cost you a penny to enjoy.