1) Engine oil doesnt get to temp, so acids and moisture don't boil out.
2) Exhaust doesn't get to temp, so moisture causes it to rust faster.
3) Engine runs richer at cooler temps, so you're getting worse MPG.

Those are 3 major downsides to running an engine too cool. A new thermostat is well worth the investment, and do a coolant flush while you're at it. 10 year-old coolant should be changed by now