Changing things completely here, but I'm always irked that Minecraft rivers are at flat sea level, meaning there is no way they could flow in any direction. I know a few mods have attempted to make them look like they flow (Streams, was it?), but having riverbeds tie into the adjacent biomes' average Y-levels would feel better to me, especially if they would pool up as lakes in areas where a natural flow would stop. Not sure if it's possible, or the toll it would take on generation, but it irks me all the same.