One fun fact about allegiance switching is that there was a pretty good deal of it, and those who switched traveled a decent amount to find a new chieftain/farm to work for/on. This is why there aren't really any language dialects in Iceland today! Too much internal migration.

Actually, a lack of forests may have been that biggest reason for this. Travel was often difficult due to rivers and mountains, but it seems forests contribute more to the formation of dialects ... I do not know why that is.

Regarding climate/geography, Iceland would have been different because it was colder and farther from other sources of goods than, say, anywhere in modern-day Denmark. The fjords would have been frozen longer, so you were trapped at home longer, and there was less diversity in the resources that you had at hand.

The climate here is warmer than in the most northen part of Norway, thanks to the gulf-stream. And during the Viking age it was even warmer than it is today, it got colder a few centuries later (this is one of the reasons for why the colonization of Greenland failed).

The fjords here do not freeze, except for the Vestfjords, and even then it is only during the most harsh winters when we get a lot of drift-ice from Greenland. Sailing was greatly more popular than it is today, it is sadly something of a lost art :(

While Iceland does not have many of the resources Scandinavia has, it does have it's own perks. For example, Vikings were trading seal skin, hunting falcons, and narwhal horn (they are actually teeth) to European royalty long after the end of the Viking age.

You are spot on about the horses. And you could ad the sheep, goats, chickens and (my personal favourite) the Icelandic dog to that list. The horse is often called "man's best servant", but the dog is still man's best friend :)