Welcome to AMS - my little insignificant corner of the world to which I'll scribble ideas and suggestions for the game Minecraft which is currently in its Alpha phase. Why here? Well, because I don't have much time to wander around forums. Instead of spamming posts or forums I'll just collect it all here. If Mr. Persson would happen to see it then it'd be great. The bottom line is however when I get ideas I must get them out or I'll burst so it doesn't really matter - I have to get them out anyway. So, let's get started...
Tile Graphics Variations
Each block gets a random number (say, from 1 to 10) attached to it at the time of birth. The tile set artist can then define several (in this example up to 10) different variations of graphics for the same tile. When the game is about to render a block it will check if there's an entry made by the tile set artist at the index that the previously mentioned random number points to. Of course, the artist doesn't have to define more than 1 graphic for a tile, but those artists with really awesome ambitions can add up to 10 variations for all tiles. I would guess that this wouldn't be a very complex feature to implement and it would easily give artists a lot to work with.
With variations, think grass that's not just green but might have small flowers in the textures or spots of brighter or darker grass. Think brick blocks, with evenly colored bricks or some with a lone brick of a darker color. Mineral ore blocks that looks slightly different than its neighbours of the same type.
Simply craft a certain block that when used will give the player a list of all other such blocks in the world, and the ability to name the currently used block (think something like waypoints in Diablo II except you can build them yourself). Select one of the blocks in the list and get instantly teleported there. Personally I think this block should be really expensive. Say, 1 diamond block in a circle of obsidian or something. Nothing you just build after the first 15 minutes of play. There could even be a cost to use the waypoint once (say, you need to put in redstone to power it, or coal, or milk...) increasing exponentially with distance. Exponentially because that will force players to build several in a sort of line to be economic ;-)
I got this idea before Notch mentioned doing something similar to the Ways from Wheel of Time which admittedly sounds really freaking nerdily awesome... but is probably a whole lot more work, but now I'm just guessing. Then again, who says there can't be more than one way to "fast travel". This might be the easy but expensive way, while the Ways would be the less expensive but butt-hurt difficult way.
Lava Powered Furnace
Just because it looks bad-ass, I have placed all my furnaces at the end of a stream of lava. It's like Ironforge, or whatever. From the moment I did, I had always wished that the furnaces would actually get power from just being in contact with lava. Notch did introduce powering furnaces with buckets of lava which is cool, but rather quite expensive in my opinion. Then again, getting infinite power from a pool of lava might be too cheap. Uh... balance it by making adjacent-to-lava smelting go slower? I don't know. I do kind of like the need of going out to find and mine coal though. Then again, you'll always need coal in large amounts anyway to make torches and you'll not have lava around at all times either so you'll probably power most of your furnaces with it anyway.