I think a new gamemode, History Mode, should be added as a mod or an actual implement to Minecraft - as it would relieve many Minecraft players due to the 1.1 terrain coding change - to stop the "boring terrain" which is terrorizing the veterans of the Minecraft community.

History Mode, its purpose quite obvious as its name is History Mode, will actually downgrade the world it generates to the desired version of Minecraft, while maintaining the compatibility with the current Minecraft version. Since it is well and truly downgrading the world, even all of the textures, maybe not bugs, and mobs retain their previous sound effects and AI coding! As Minecraft worlds do not contain a minecraft.jar, they go to the one in the "bin" folder in .minecraft. But instead of that, it will contain a minecraft.jar and a bin folder of its own, but, as there will be one in every world, there will be no bin folder in the initial .minecraft folder. Instead, all texture packs and mods are directly downloaded into a texture pack/mod folder in the world's folder and there will a be a minecraft[version].jar instead in a bin folder.

As there would be no initial texture pack section in the Main Screen, when selecting a world in the single player world selection screen, there will be an options option. When clicked it will open a screen exactly like the one in the title screen but for the world which you are changing texture pack for. As for HD patching mods such as Optifine, they will be installed with their whole ZIP file in the .minecraft folder - not as the bunch of files just thrown in but the actual ZIP file in .minecraft. But for MCPatcher, as it patches automatically when "Patch" is selected, it will transfer copies of the files it adds to .minecraft and minecraft.jar, but to the worlds' minecraft.jars instead.

Why this should be added as a mod or gamemode:
Terrain,
many Minecraft veterans will say that they preferred the old terrain as the new terrain is "boring". Extreme Hills biome, is probably the most missed as it always had "the most beautiful" rivers and waterfalls streaming from the mountains with "awesome" floating islands that appear to have been a pre-chunk error, but people like those errors. At the moment the terrain has really common Taiga biomes, along with Jungle and Desert being very common as well, and most people reviewing the new terrain in their tutorials on Youtube have said that those biomes seem to be the laggiest to be in and apparently in Jungle biomes the chunks do not load very well which really bugs people, including me.
1.4 sounds,
these are what bug 75 out of 100 people. Sand's new sound is probably the worst, but many people say they like the other sounds, since it sounds like you're trudging around in deep mud and you're about to slip over in black ice hidden in the mud. Plus it's very irritating because the sound isn't exactly smooth like the old sand where it was nice to hear in cheap headphones, but then now it's quite hollow and echoes a bit.
Beta,
beta. Just Beta. Minecraft Beta was probably the best time for Minecraft to be played because the terrain was nice, the sounds were smooth and it was just a lot more organised than now.

History Mode would be great fun for me, and definately for the veterans of Minecraft!