Haven't played since the end of Season 3, what has changed?
I stopped playing League what feels like ages ago, during Preseason 4, at least for the most part. I've occasionally logged on the PBE around the times of new champ releases to try them out with the exception of Jhin, who I didn't much care for because I don't ever really play ADC. I used to main mid, support, and top. Some time mid season four, I tried learning a bit of the jungle to play Rek'sai, who I've fallen in love with, so the few times I have played since S3, it has mostly either been Rek or a new champion.
I've seen a lot of things about various jungle updates, people complaining that ADCs might as well not exist any more, itemization updates (I have no clue what to build for a midlaner any more), and loads of other stuff. So would anyone mind filling me in on what I've missed the past three seasons? What changed? What are the cool kids building these days? Why are my friends that still play complaining about ADC? Anyone mind enlightening me?

