Alps Coal plentiful, windy noises, a few flowers scattered around.
Autumn Woods PUMPKINS (and cedar trees explained below)
Extreme Jungle GOLD
Glacier spawns as an island?
Ice Wasteland IDK why but it spawns strange ice 'trees' all over. I would move this to 'addons' Mobs can't spawn here, there is no reason to go here (ore, items), and it's not aesthetically pleasing. Also you can't really even mine here lol.
Marsh Awesome addition.
Meadow More flowers. (explained below)
Mini Jungle This and green swamp are very very similiar to me. I can't really think of a way to separate the two.
Mountain Desert Water should be teal instead of that Notch blue maybe? Like http://miriadna.com/desctopwalls/images/max/Desert lake.jpg
Mountain Ridge Iron plentiful here
Mountain Taiga It looks a bit like the alps without the nice trees.
Pine Forest Add those cool trees from the alps into the mix.
Rainforest Looks like 'forested hills'
Temperate Rainforest Snow
Bayou needs uncommon cobble/mossy cobble/wood shacks with skeleton spawners in 'em. Perfect biome for skeles since the water and trees protect them during the day. I'd get bones here.
Bog damn this biome is annoying to traverse through. I love it. Collecting that mud for traps or barriers would be why I'd go here.
Dark Forest This biome is the spiders' domain. Both blue and black spiders inhabit the trees of this biome. I'm not really sure where the spawners would go. High in the trees would be cool as long as the spider can live the fall to get to you. Potions would be the reason I travelled here.
Deadlands Rather than all mobs, only creepers should spawn here since it's the only biome suitable for the creepers sad existence. Gunpowder is obviously the reward for traveling here.
Deciduous Forest Very generic for an add-on
Flower Field This is cool but I would actually like it in the main mod. Meadow.
Gravel Beach It's not really a beach and too similiar to wasteland to put in thed main mod, but it's always cool none-the-less.
Maple Forest I like it - Perhaps replace Automn Woods in the main mod with this. The addition of the tall trees along with pumpkins would really pull it together in my opinion.
Mystic Forest I loved this add-on so much but there is no reason to be here unless you're trying to get mushroom blocks. Perhaps the odd diamond could spawn near the surface. So much possibility here - I just can't think right now.
Oasis Only have the palms spawn on the grass around the actual Oasis.
Prairie Main mod.
Tropical Rainforest Love the Melon addition! would go with pumpkins in the automn Forest. Main mod.
Tropics Love this. But paradise should stay in the add-ons.
Wetlands Main mod
Wheat Field Main mod.