Given we're not setting the projection coordinate transform tag I suggest we limit this to GeogCS. I'd put in:
```python
if coord.coord_system is not None and \
        not isinstance(coord.coord_system, iris.coord_systems.GeogCS):
    raise ValueError('Unsupported coordinate system {!r}. GeoTIFF export is '
                     'limited to GeogGS.'.format(coord.coord_system))
```
or similar. As it stands I can have a coord with a GeogCS coord system and still get a failure if the parameters (e.g. earth radius) don't match. The resulting error message will make no sense.