There are a lot of abilities and interactions that no longer exist. I don’t know about all of them, but let’s name a few.
Remember when using Q with Garrosh would pull the enemy hero to you? That one button usually meant a kill.
No one would ever do the E+Q combo back then, because it would pull the target back to you and away from your team. But it was really good peel by itself.
Li Ming talent, i forgot the name, that pulled heroes in the center of her orb and that allowed her to combo w + q and insta delete a guy.
I remember when “tiers” of talent choices had to be unlocked by gaining levels on the hero in question…
…It felt weird.
I remember when Abathur could use the ults of heroes he cloned.
Imagine double Genji sword ult today.
I remember when Kerrigan could summon zerglings.
Remember when Tyrande could take a sniper build and sometimes murder people from the other side of the map with her owls?
Funny times.
Wait seriously? 
Tyreal’s Cast Aside talent… one that defined him. now gone, lost to the ages.
What did it do? 
It cast heroes aside when you used smite on them.
It was her lvl 7(?) W talent in beta.
Remember when Alarak had a talent which only function was to decrease the mana of E on 10?
Nova had a talent called Railgun that allowed Snipe to pierce enemies.
Nova (and a few other heroes) had a talent called Gathering Power that increased her damage by I want to say 5% per kill, stacking to 25% I think, until she died. It was basically Tracer’s untouchable talent but better.
Nova had a talent called Ambush Snipe which increased the damage of Snipe by like 30% if she used it from stealth. Those two talents combined were the cause of much whining from QM players. Anyone who complains about stealth these days makes me laugh, as they have no idea the pure havoc old Nova was capable of wreaking.
Kerrigan had a talent called Adaptation that allowed her to use Ravage to jump to allies. If timed correctly, she could fly across the map with Falstad when he used his z.
Tyrande used to have a talent called Searing Arrows that increased her aa damage for a few seconds on a medium (~25 sec I think?) cd.
Ess of Johan. It was extremely powerful because it was a level 4 talent and Nova was totally destroyed that same patch. If Nova hadn’t been essentially removed from the game that patch, Ming wouldn’t have been nearly as problematic.
Remember when Varian’s Warbringer used to replace the slow with a stun, and was so OP it made the original form of Shield Wall a meme pick?
Remember when Zeratul could have the passive effect of MotN and Void Prison at the same time? MotN’s damage boost used to be a level 7 talent called Combo Slash.
Remember when Murky’s Pufferfish had a hit counter instead of a health bar, so Mages literally couldn’t destroy it in time but Tracer could delete it in 0.3 seconds?
Remember when Butcher got buffed a tonne and was insanely OP for about seven days after Garrosh came out?
Oh, well, you can do that with Holy ground now.
When Possession was a Heroic for Sylvanas - not a level 7 talent. 
I remember beast mode Nazeebo with vile infection baseline, spiders that were not a skill shot, if you hit your target or not they would chase the nearest hero. He also had a talent on exploding frogs that would release more spiders!
His Gargantuan would do 100% more damage to buildings. Gargantuan had a huge leash range and would chase heroes and follow you, at 20 it would stay summoned until killed and follow you, including in vehicles like the DK and old GOT vehicles.
Yeah, that was release Varian, wasn’t it?
My contributions:
Remeber when Azmodan could keep channeling All Shall Burn for as long as he had mana?
I got so many kills by coming in from behind and channeling as soon as the enemy spent their CC, so I basically forced them to choose if I kill them (as they try to run past me to retreat while I fry them), or they get out of my range the other way, which basically meant running into my allies and the towers… And my Globes 
Remember when Towers had Ammo and almost every Specialist had an early Talent that let them remove ammo with basic attacks?
As soon as a tower was down, I could push the fort/keep with Gazzlowe, because it would take me less time to run the other tower or the fort/keep itself out of ammo, than it would take the structure to destroy my turrets.
With Zagara’s Medusa Blades, I could sometimes drop the ammo of both towers to 0 before they took down my roaches.
I still miss Zeratuls double bombs talent and the old stealth.
I also very much miss Diablo’s fire stomp going in a circle as per lore and actually doing good damage. There was even a time when his pure fire stomp build was good! I also miss his old charge. It was also cool when you could consume souls to get HP. I also kinda miss the speed demon talent, many people considered it bad but it gave you a speed boost when you needed it most (after being stunned or rooted). If you can’t tell, I miss old Diablo builds a lot…
I miss the talent that gave Xul health each time he raised a skelly.
I may be alone here but i miss Butcher when his meat maxed at 30-35 and you lost them on death. It was big risk/reward play style i really liked. If you died it wasn’t all that hard to get back to max if you were good with him.
I really miss old Tassadar! He was basically the only hero i used to complete the support quests. Now i hate that quest as much as i hate the Overwatch quest!
Oh and i also liked it when Sgt. Hammers non siege basic attack did the same damage as when it was in siege mode and siege mode was strictly for splash damage/range. Was a lot more dynamic fighting style as you could participate in smaller fights out of siege and not feel too gimped if ya didn’t need the splash or range.
That was the best way to kill a Valeera. It would keep channeling on her no matter what. It was funny to see them cloak then" Wait what?" use smoke bomb, “Wait what?” then die lol.