Mythics have only 2 fixed affixes instead of the whole item. However you can now also enchant mythics, so effectively you can pick a 3rd stat, but it makes them RNG for the 4th stat. Also not all the unique stats that were on the slots were preserved(Heir of Perdition used to have crit chance, lucky hit chance, movement speed and +2 core skills, now it has movement speed and main stat, with the 2 other affixes being only generic helmet stuff so can find some +skill but won't be able to get crit/lucky%). Generally speaking natural mythics were nerfed across the board as a result.
That said there's a bunch of other changes, for example you can upgrade Uniques into Mythic Uniques(30% bigger unique affix), you can enchant uniques too and they also come with 2 preset stats(outside of shitty choices, it makes them a lot more consistently good).
The biggest complaint I've seen for this new season is it isn't really a new season, just the same as the previous with some minor changes and a proper seasonal mechanic(unlike the previous which had nothing due to being a new expansion), and they made sure to put 2 bullets into the back of sorc's head but only significantly buffed druid(which is still likely to be worse than Barb and Rogue). So mostly more of the same.
I'm going to play it cause I have nothing to do until next week when the games I want to play actually release, so I plan on just getting the free shit and season done quick and then forget about it, but yeah not an interesting season.