When did you stop playing?
The game is still the same pretty much. Instead of 4 Acts there are now 10 and a single playthrough instead 3x the same 4, with the negatives of -resistance happening after finishing act 5 and 10. Story evolved from escaping Wraeclast, killing the High Templar and their god, to going back to Wraeclast and getting assistance by Sin, another God and brother of Innocence who wants the world to be free of gods, so you get to kill more gods. After that, you have the map content which is the original endgame, where you complete maps with certain challenges to unlock the Elder and Shaper and in the end the Uber Elder fights (with Uber Elder being both Elder + Shaper present). Uber Elder is the hardest fight in the game, besides a very small amount of other stuff. Endgame in itself isn't just doing maps anymore though, since there is now Delve, which is basically a unlimited dungeon that gets harder the deeper you go for example. Atziri / Uber Atziri with the Vaal Fragment pieces, Breachstones, Beachhead/Harbinger, Betrayal, Uber Lab runs, Bestiary and Incursions of the top of my head and a lot more content in general.
Too many major changes in the past and present. Right now melee is getting entirely rebalanced and reworked to be more modern and better to play. A lot of the clunkyness that plagued it get's removed with 3.7 which launches on friday/saturday depending on where you live. All in all a lot of systems have changed for the better most of the time.
Gems are still locked to level 20 with 20% quality at max. You can use a Vaal Orb to get 20/23% or 21/20% gems. There are also other ways to improve gem levels via gear (+1 to socketed gems is an affix for example), which effectively can increase levels way higher than it would be possible. There are no "tiers" so to say. New gems get introduced every 3 months when a new league hits.
If you mean Unique items: every update brings new ones as well as a shitton of other items (divination cards, possibilities to craft etc.). Rares are still the best way at the very end to gain even more power instead of wearing uniques. Also, you are entirely wrong about "lower level gear not being redundant", because itemlevel of the gear itself increases the affixes' tiers. That means a ilvl 86 item can roll tier 1 affixes and everything below that can roll tier 2 affixes at best. Meaning: Tier 1 = 100 life for example, Tier 2 = 90 life at maximum roll. Obviously you want to have as much gear as possible with Energy Shield if you play a Energy Shield based character. You can even go further with Shaper/Elder items, which have entirely different pools of affixes aswell.