Okay... a couple of things I would like to see/see changed:
1) started a new world "jungle book" spawned next to jungle. Very nice looking. Jungles are very hard for AI to navigate. Monsters got stuck a lot. Maybe instead of using low trees for ground cover, it could simply be some sort of obscuring bush comprised entirely of leaves?
2) Ocelots are not really very rare. Found 2 of them almost immediately. No trouble catching up to them. If it were possible, I would have the ocelots able to move through leaf jungle blocks. It would have made them much hard to see, and much harder to catch.
3) Get the sheep out of the jungle; they get stuck, are hopeless, and do not belong in that biome.
4) Pigs would be good in the jungle.
5) Prevent jungle biomes from generating sand beaches. How about gravel or just mud?
7) How about, instead of moving slimes to the jungle, or in addition to, there were hostile jungle cats (jaguars, tigers)
8) Jungles would seem to be natural habitat for creepers.
9) Would like to see glass blocks and frames able to be dyed.
10) would like to see upside down stairs, for the undersides of things.
11) would like to see half-blocks able to be placed vertically.
12) Vines should be climbable WITHOUT a backer block.
13) Would like to see ability to enter text on blocks or half blocks.
14) Lots of ocean biome that is virtually completely ignored. Let's get some mobs there!
15) Ability to build actual ships to explore ocean biomes. Could use furnace/wool sail.
16) Wind effect (possibly connected to clouds) that is directional and can be tuned (for servers)
17) half blocks for wool.