1 and 4 (part 3 of the loops)
Most of the loops in 5 seem to be coming from season 2 and season 3, but there are still a few standouts from 1 and 4. I'm doing this because it's hard to find anything that's actually original in season 5.
1 is like 5 predominantly in how it relates to Mike and El
-A child close to Mike goes missing
-Mike makes promises to El about what they'll do together after they defeat the bad guy (and again when they get Will back)
-Mike says El looks pretty in her new outfit (which is a weird comment because it's just a wetsuit, not a pink dress)
-El goes into the bath after promising Joyce she can find Will and Joyce thanking her.
-El has a scene in the tank with Hopper that matches her scene with Brenner in 1
-El's goodbye to Mike and his reaction is almost identical
-Karen opens the door to let Mike and the party know it's time to finish their D&D game, the way she did at the start of the series.
-Lots of Eddie related homages in clothes and comments "It's the tone." "Bad news first, always"
-Running up that Hill is played again to wake Max up. A lot.
-Will treats the demos the way Vecna treated his victims
-Murray gets into a locked military zone by pretending to be someone he's not in a truck.
-Max and Holly going through memories like El in the Nina was going through memories
-Attacking with Molotov cocktails and guns again
-Robin is giving bad/wrong relationship advice again, but still from a good place
-Robin figures out a mind/music connection (in 4 it's that music can rescue you from Vecna, in 5 it's using album covers in place of Max, Holly and Will to explain how they're trapped in Henry's mind. )
-Steve's six nuggets are mentioned again
-Purple Palm Tree Delight is mentioned again, as well as the Emerson lie.
-They are driving around in a van again. WSQK instead of Surfer Boy
-Steve and Robin working together again (like 3 and 4)
-Max and El are fighting Vecna in a memory again-his this time instead of hers and with Kali and a pack of kids along for the ride.
-Vecna in person is fairly easy to defeat. Again.
(What's really fun about this is there is NO mention of what Mike and Will went through in that van or the painting. No resolution to the "From Mike" fight and off love confession to El in the tub. NO REFERENCE to the Will, El, Mike story line from season 4 for AT ALL in season 5 except that El knows she isn't a "monster" anymore. It was like it never happened.)
If you combine that with what I found in 2 and 3 above, there really isn't much about season 5 that isn't a reheat other than:
-The wormhole explanation for the UD
-The walls (Pink Floyd posters, quarantine fence and fleshy wormhole wall)
-Will being hopeful, even though that was very short lived
-A Wrinkle in Time references
-Mr. Whatsit version of Henry/Vecna/One
-the mine memory and the high school and play memory (two of which were never followed up on)
-Karen and Mr. Clarke being included in the supernatural aspect of the story line.
-having to follow the 15-20 new characters from Dr. Kay to Akers to Derek