Honestly, I think biomes would be really, really interesting. It would provide a good reason to explore; instead of finding more grassy hills and loose forests that look nearly-identical to what you left behind, you could find interesting areas - massive mountain ranges, vast deserts, etc.
Smooth transitions aren't dependent on an infinite plain, near as I can tell - you get some region X, some region Y, and some transition boundary X --> Y that has to be dealt with. Having an infinite plain just means you've got boundaries on all sides of all regions. Making the boundaries smooth is the problem Notch ran into last time he tried to implement biomes in an SFU. Personally, I'd be happy with a 'ragged edge' - a desert-to-grassland would have a jagged border, perhaps with a few sand tiles scattered into the edge of the grassland and, vice versa, a few grassland tiles peppered into the edge of the desert. It wouldn't look 100% smooth, but it'd be close, and I don't think smooth edges are really suited for the highly-blocky art style of Minecraft.
Seasons would also be cool, though. Summer could be more-or-less what we have now; autumn could have leaf tiles decay even while still connected to a tree; winter could have snow fall and water freeze; spring could have leaves regrow (though I guess there'd need to be some way to distinguish natural trees from "raw logs" used in construction, else your stylistic house would start sprouting leaves!).