So the carriage at your house not only provides free travel, but it can take you to places you've never been. Some example for me currently are Darkwater Crossing, Solitude etal that are on my map, but I haven't discovered yet. Anyone else think both of these might be considered RP issues?

I don't see why this would be a problem. Your carriage driver surely knows Skyrim's geography better than you and traveled to all these places before. Think of it: when you travel in real world, you don't need to have visited a certain city before to buy a plane ticket and fly there. It's the pilot's job to know the way, not yours. It's the same with carriage drivers.

What would be different about that than hiring the carriage at Whiterun to take you to Solitude? For new characters I always take the carriage for my first visit to a city to avoid "complications" involved in walking. Wouldn't that carriage driver know the way even if you don't? I don't see the issue here. If anything, it makes life easier for RP players who don't fast travel to have transport to places other than the big cities.

Agreed. Your taxi driver in New York is likely to know the area better than a tourist.