You need to be able to cultivate red mushrooms like blue ones, because cutting the grass on the surface world just isn't producing enough. It takes 100x more effort to get red mushrooms than blue once you've found a plot, which doesn't make a lot of sense. The red ones should grow underground, somehow, too. Just growing on the surface makes the really rare if you do any digging, and they just should be growing in a cave. They're mushrooms!
I think I've probably given at least 4 worlds the "12 hour treatment" now with different friends, without really going on beyond meteorite.
Really, the game's content needs time to catch up with our appetite. A few well thought out accessories could help rebalance the game in some places, like an accessory that just gives you increased pierce damage, allowing you to keep older less effective weapons and keep them semi-useful.
Rarity....I dunno. I've played probably 12 hours in the last two days with up to 3 people or more and we've found 1 or 2 chests. Granted, we haven't been tearing the world apart looking for them, but high rarity can really hurt in the early game. On the other hand, some of these awesome rare drops are way too common. It's hard to really say what needs to be super rare, when Shadow Orbs are a dime a dozen and a guaranteed item from the ~Demonite Tier.
The Molten Pick needs a nerf, just because the jump to a 100% power pick is too huge. TBH, they should have left all the "100%" style items out of the game and built their way there. The only way to rebalance it is to retcon those items.
Hell in general just needs to be spicier for the amount of effort it takes to get there. A few drops, the forges, the hellstone, but there's really no surprises there, especially in the topography.
There really needs to be more isolated sites inside the mantle of the world. Little temples or graveyards or small, one off things that are pretty evenly distributed. Nothing the size of the UJ. There is a point at which digging becomes completely pointless, and it would help preserve the desire to really check every nook and cranny of every world. (Or cheat and go clean it all out.)