It is a lot harder to get cold water species unless you can capture them yourself. There are very strict laws in some areas of north america on what local species you can have and what permits may or may not be required.
That being said I've seen some very nice coldwater tanks. My favourite is Coldwater marine aquarium Blog
. It was running for 2 years and he shut it down when he moved just over a year ago. Tropical temperatures remain somwhat constant year round, while temperate climates go through temperature swings, high in the summer, low in the winter. One decision you have to make is whether to keep the temperature constant or go through the seasons.
One of the main differences is a reef tank replaces corals with macro-algae. There are coldwater corals but they live very deep on the ocean floor. The macro algae also replaces some of the colour you lose with colder species of fish.
I plan on starting a cold/temperate aquarium, but I wanted to get my tropical freshwater tank running first. Two big things needed for the temperate are the chiller required to keep the tank at a low temperature, and depending how low you want to go, a custom built tank so that it's insulated a lot better and easier to chill. If you don't do a custum tank, then at least do acrylic, as it insulates a lot better then glass.