If your car is sitting at higher temperatures when moving slowly, the obvious thing to check is the cooling fan. Having said that, slightly over halfway wouldn't worry me at all. The gauge isn't precisely calibrated, so small variations in the sender and gauge will lead to some change in where the needle sits for the same temperature. Variations in the thermostat will also lead to changes in the exact temperature it holds. Thermostats, water pumps, bad radiators and coolant levels are unlikely to be at the root of temperature variation at different speeds - if there is sufficient cooling capacity at high speeds then there should be no issues when at idle or just above, provided there's sufficient airflow over the radiator.
You really don't want to run the engine as cold as possible at all times. Cold engines suffer significantly more wear, because the clearance between moving parts is tighter and you also have issues such as fuel condensation on the cylinder walls. What you want is the engine to reach its designed operating temperature as quickly as possible and then stay there. If it was really bothering you that much you could change the thermostat for one that opens at a lower temperature. Personally, I'd potentially be more concerned at the low temps you're seeing, not the high temps.
As a side note, I disagree with Nuffles and GonvilleBromhead saying the gauge doesn't show anything useful. It's very useful to see the rate of change, or just to see abnormal indications. I've been having problems with the cooling system on my car, and I know exactly where the needle should be. If it rises above that I know immediately that something isn't right, but if I can't stop immediately I set the blowers to max hot and then I can watch what effect that's having. Sometimes it's enough to stabilise the temperature at an elevated level until I get home, sometimes it isn't. Sometimes the air bubble will bleed itself and the temperature will drop right back to where it should be and I can turn the blowers off. In each situation I can prevent it ever reaching red line temperatures. A simple light wouldn't let me know anything was wrong until I was already at the red line and I'd have to pull over immediately.
If that's still not enough of a reason to have a gauge, on one occasion I had a coolant hose develop a pinhole leak when I was 7,000' up a volcano. I noticed the gauge was climbing and patched things up before losing too much coolant, so I was able nurse the car 50 miles back to the city where I could get a new hose. If I'd had no indication of a problem before the red line was reached I would probably have had to have the car recovered, at a cost of a few hundred dollars. Gauges are great, if you monitor them.