One thing I love in minecraft is exploring giant underground caverns, but I must admit what I would love to see is some more biodiversity underground.
While I know it is not realistic to run into a mushroom forest underground when mining, I am already on the lookout for skeletons with bows riding giant spiders, so I am definately getting a fantasy vibe going on.
And since the days of Jules Verne (journey to the center of the earth) how could you have massive underground worlds without fungal forests.
Fungul forests would require fairly large underground spaces to appear, they should be giant multi-block mushrooms the size of a small tree:
   
These would have their own rules to grow and place similar to trees (such as must be on dirt, must be near water, cannot have sunlight), but with the reduced likelihood of wide open spaces, a mushroom forest would be rare (as sunlight should destroy them). Edible mushrooms should be more likely to spawn around a giant mushroom. Harvesting should give you a red mushroom (which you could plant and hope turns into a giant).
There should also be the odd patch of glowing mushroom, giving light equal to a red stone torch. Moss should also be able to appear near glowing mushrooms, turning stone to mossy stone and cobblestone to mossy coblestone in a similar manner to how dirt becomes grass (requiring low light, neither high light nor lack of light). Yes this would mean more mossy cobblestone: but really, why should mossy cobblestone be rarer than diamond? thats going to be short term anyways.
And of course this also means underground creatures should exist, I think bats have been suggested (small annoying 1 block flying mobs that have glowing eyes, giving false positives that there may be a spider nearby). Perhaps creatures such as rats would also work.
Now I don't mean that caves should be as bustling as the surface world, by and large they should be dark and empty tunnels. But their should be the odd sprig of life down there, everynow and then you should tunnel through a piece of rock and stare in a vast chamber full of giant mushrooms, dimly lit by lava and/or green glowing mushrooms. Bats flutter about and rats (or giant beetles, or small slimes, or all of the above) crawl around.
This is a fantastic idea. I mean, I pretty much live underground, and I'll be the first to admit it... The cave system is pretty boring after you have played it more than once.
The fungal forest is a great addition, along with rats and bats... So finally the regular world will have a freaking flying mob. And the glowing mushrooms... In real life, I want to live in Japan where these grow. They are amazing.
But I would say that the giant mushrooms should drop... um... "Mushroom-lings"? Kidding, they should drop "Mushroom Spores", and the drop rate should be low, if not rare.