So, since I use Classic Mechanics in conjunction with Nostalgic Tweaks I am going to only suggest things that are in neither (and which are unlikely to be added by Nostalgic Tweaks).
Right click a vehicle/rideable entity to dismount (pre r.1.6 behaviour)
All animals use wheat to breed and follow the player if they hold wheat (Beta 1.9 Pre release 2 behaviour)
Unconnected Glass Panes and Iron Bars being shaped like a + (plus) rather than a • (rod) (Beta 1.8 - R.1.9)
Shift + F2 resulting in a 36450 x 17700 huge (tga) screenshot)
Changing Computer Time speeds up game (wouldn't know how to do this (Pre Beta 1.6))
Leaving Inventories drop items that are not firmly in an inventory slot (Pre R.1.17.1)
Experimental Mob Spawning -> Mobs could spawn in higher light levels the farther you went down (Formula: 16-Y-Level/8, so at Y-Level 11 it would be 16-11/8 = 14,625; Alpha 1.2.0 - Alpha 1.2.1)
Zombie (-Villager) damage increases as it's health decreases (add date unknow but removed in 13w21a)
Elytra repairing with Leather instead of Phantom Membrane (R.1.9 - R.1.13)
Elytra not being boosted by firework rockets (R.1.9 - R.1.11.1), Modern, Experimental -> Gliding does not decrease the durability, using a Firework Rocket uses up 3 durability, decreased rocket boost (from 30 blocks per second to 26) and Elytra breaks upon reaching 0 durability (1.18 Experimental Snapshot 7)
Enchanting Table not using Lapis to enchant (Beta 1.9 Pre 3 - R.1.8)
Enchanting Table not displaying potential enchantments (Beta 1.9 Pre 3 - R.1.8)
Enchanting Table using all of your levels (Beta 1.9 Pre 3 - R.1.8)
Max Enchant Level being Level 50 and having the fields randomized (so you could get 16 on top, 14 in the middle and 36 on the bottom (Beta 1.9 Pre 3 - R.1.3.1)
Hope you enjoy these suggestions. Now obviously I do not expect from you to add any of these but thought you might want to know some of these.
So, since I use Classic Mechanics in conjunction with Nostalgic Tweaks I am going to only suggest things that are in neither (and which are unlikely to be added by Nostalgic Tweaks).