S3 made AP burst weaker (generally) due to unintended effect of black cleaver making everyone have close to 0 armor so we all stack HP instead... So I personally count HP as my mres, but I generally just tinker around in normals so people are still stuck in the AP Burst mid mindset and build is very similar to S2.
I personally think this is the season people need to run either AP tanks mid or AD casters...but I haven't seen a lot of it, but again ... I generally stay out of ranked (way to much QQ, I play a game to have fun)
So essentially everyone will have 50-100 MR and 100 Armor (bruiser wise) and build HP. Tanks a bit more, and carries will still be glass cannons.