"Umm... Grod, I realized you're a hearty barbarian but, are you sure you want to go out in this snow storm wearing nothing but those boots and a loin cloth?"

What constitutes as heavy clothes?

Thick clothing that would be worn out in the winter snows. Usually leathers, furs or multiple layers of wool. Think the type of clothes that Inuit tribesman would wear. Lighter materials like silk and linen provide little protection from the cold.

can someone explain how this is uncommon but a ring of cold resistance is rare?

You can wear multiple rings but only one pair of boots, making this compete with other useful magic items. Also outside the difficult terrain, the other benefits are redundant  (as cold resistance pretty much let's you ignore most cold environmental effects)

A must have in Icewind Dale. I like how you can be practicality butt naked but still be fine in −50 conditions if you wear these boots.

Cuz one are boots and the another is a ring, it’s smaller and more convenient I guess.

nope, there's some weird choices in the magic item system, like how a flametongue is rare but a frostbrand is very rare despite the flametongue being strictly better.

not strictly better, frost brands give resistance to cold, and flametongues dont giv u a resistance to anything.

It's resistance to fire damage while you hold it, but yeah you're right, I totally forgot about that.

Fewer creatures resist cold than fire.

true

You will find these more often than a ring of cold resistance... simple really

of course I made a campaign solely made difficult by snow terrain and the freezing temperatures just to find out my friends all had these :|

In my home campaign, these also sprout a pair of apple-bottom jeans for the wearer. 

No weight or cost? 

These wouldn't be sufficient to live in Winnipeg.

Cool!

It'd be fun not to have to worry about clothing in the arctic with these boots. Think of the savings!

We tested them on Mars as a proof of concept. We'll need to test them in Winnipeg to push them to their limits.