I love Minecraft. Ever since I first gave it a whirl, I have thoroughly enjoyed goofing off with it. Thus far I have really liked just about every aspect of Minecraft, save for some of the oddness of the Biome inclusions. In terms of bugs my biggest complaint right now is the lack of falling snow to refresh Ice and Snow ion the colder biome types.
Being a fond of themed maps & areas, my favorite Minecraft mod is the BiomeTerrain Mod by Bucyruss. I absolutely loved the ability to get on-demand desert biomes (one of my favorites) as well as all the other freaky stuff one could do with the terrain generation parameters. If there was a single fan-made mod that I would like to see included in Minecraft, it would be this one.
Ideally such options would have some kind of in-client interface to adjust the settings; though for an expert user using the text configuration files would not be a real hassle. I hope Bucyruss comes back and works on this mod again at some point. While some other fellow has picked it up (being paid to do it basically) I really liked the direction the original author was going in. Hopefully nothing bad happened to them.
Hooray for Sandstone! Now making sand castles will be even more fun.