Even at a 15 degree tilt you still have seasons. The difference of temperature between them is lower and the effect of the monsoon is also lower. The poles are colder during the summer but hotter during winter. It means that the tundra would stretch to lower latitudes since they need to have at least one month with the temperature over the freezing point and that condition is harder to archive. The ice caps will also cover more land. The temperature there never go above 0 Celsius. Deserts would be found closer to the equator but could probably cover less land. Rain forest and desert could be replaced by the steppe in some areas. (I'm talking in comparison to Earth)

The south can't be closer to the star all the time. Unless your planet is tidal locked to the star, the movement of the planet is similar to the movement of planet Earth.
But you are right, the north is colder but it's because the % of water is greater. Also, the north lack a continent to block the polar current that circle around with it's cold water. In the south, the cold currents and hot ones clashes more frequently, allowing a better distribution of temperature. But not in the north. The cold water just stays there just like Antarctica. From that, we could say that the northern mid latitudes are hotter than those in the south because it's not coled by the polar waters. Or not as much as in the south.

Since you don't really have land in the north, I wonder what the sea ice look like during the cold season. Ice can't form if the current is too strong. It's not very significant but I just wonder.