For the last few weeks Civilisation V always generates the same kind of boring map: flat land, very few mountains, most of the land desert or tundra. Is there anything that could have caused this? I have not installed any mods or modified the game in any way.
Did you configure it to be arid and old; or random for everything and just have a run of similar worlds?
You could explicitly set the World Age and Rainfall (from your description, the temperature could have varied a bit) to mix it up.
- World Age: Adjusts the map "smoothness", as a longer world has eroded more.
- 3 billion years: highly mountainous
- 4 billion years
- 5 billion years: flat
- Cool: more ice, more tundra at lower latitudes
- Hot: more jungles, less tundra
- Wet: more forests and jungles
- Arid: more deserts
Configure the settings to a younger earth (3 or 4 Gyr) and temperate or wet rainfall if you want to get away from what you're describing. You can do so on the "Advanced Setup" panel when you configure a game.