Mobs have more interesting mechanics in higher tiers (tier 1 and tier 2 are mostly the same though). For example, some of the tier 3 mobs shields their nearby friends so you have to kill them first. In tier 4 mobs not just hitting harder but also have some new mechanisms.
However I do think the gap between tier 3 and tier 4 is a bit high. Tier 3 is gated at level 50 which is probably fine because most people is probably able to get to level 50 pretty quickly, but tier 4 gated at 70 (all mobs are 73+) is going to take a bit longer for most people. Of course it's probably intentional because sometimes I forget that the game was just released not even a month ago
I also really like to see a sort of "respec simualtor" in game. Of course there are 3rd party websites but it's more convinient to be able to experiment something in game. It'd be nice if I can adjust some paragon points without spending gold (I'm fine with it costing gold to finalize the changes). Right now small adjustments are cheap enough so I don't care too much about it but it can be quite costly to do a large scale respec.