Did I miss anything? Did they give away any mystery skins free like they did in 2014?
First thing that comes to mind: You missed out on the anniversary. They gave out gifts; you missed on the IP weekend but you should notice an icon, Kayle, and a RIOT skin for her in your account.
I've noticed a lot has changed. Most prominently the client.
The new client is still being worked on so there are going to be a few issues and features not in yet.
What is 'power creep' ?
Here: the forums collectively decided there is too much damage and that's why all the problems in the game exist, usually mentioning keystones. It's just the latest buzzword for board circlejerk, used for evidence for any argument and yet another reason why "League of Legends is dying".
Nevermind that most of those arguments have nothing to do with the "Power Creep" design concept.
I just got back into League after a year+ hiatus.
An idea is to go back through the News section. There may be some year-in-review posts to help you out a little. Like this one:
https://review2016.na.leagueoflegends.com/en_US/year-in-review
It at least mentions the VGUs a bit down like Taric, Ryze (again), and Yorick.
Something that I, personally, would do if I have not been following the game would be to read through all the patch notes since I left. It would give me better understanding but I imagine that is not everyone's cup of tea.