2e specialty priests allow a ton more PC divine power customization than 5e cleric domains.
I like a lot of 2e's various psionics better than the few subclasses we have gotten for 5e.
In depth monster entries. I like the 5e MM and a lot of Volo's but the 2e monstrous compendium series is fantastic and I like a lot of the 2e descriptions. The 2e Monstrous Arcanas and such (draconomicon, giantcraft) are a lot of fun too.
Full on campaign settings.
Fleshing out campaign settings.
Campaign setting sourcebooks detailing one area.
Ravenloft

I prefer the 2e setting with politics between dark lords over the 5e isolated dream prisons.
Greyhawk.
I prefer 2e fleshed out FR over advanced 100 years 5e FR soft reboot where most old lore is now inapplicable.
2e God books are pretty fantastic. 5e only has a set of decent charts in the PH appendix.