The symbolism of the bear
The following is partially based on an especulative interpretation. Philip Pullman ever considered this when designing the panserbjørne, for he could have just done such for Rule of Cool. Nonetheless, the author of this text looses nothing to write this essay, except perhaps some time.
His Dark Materials is, perhaps, best known for its controversial portrayal of Christianity, but there's always an element of the series that makes them very iconic: the armoured bears. Known as "panserbjørne" (danish for, guess what, "armoured bear"; the correct plural would be "panserbjørn), as "panserbørne" in the earlier editions of the first book ("children of the armour" I believe), and as "ice bears" ("isbjørne" anyone?) in the movie The Golden Compass, these are sapient polar bears with opposable thumbs; whereas they are analogous to our polar bears or if true polar bears occur alongside them in Lyra's world is unknown, with this troper preferring to opt for the first, given how unlikely it is for the more primitive polar bears to survive with more adaptable and strong competitors. Their main feature other than sentience is their skill with metalurgy, producing armours out of asteroid metal (their "souls"; see below) and, if forced to, weapons for humans. They are pretty much the reason why His Dark Materials has more fans than people interested in philosophy, because they are the among the ultimate bad ass generators.
The reason why Pullman decided to turn polar bears into these inspiring race could have several interpretations. For one side, he probably was just being original; fantasy is infested with humanoid races, and having a sentient species that, while vaguely humanoid, would still be as far apart from man as dogs are, would probably be a lot more interesting. Again, it could just be for the "coolness" factor. Or he could have used it as a Take That! to Narnia; in pre-christian Europe, the bear was way more widespread that it is today. Back then, it had the status as "the king of the animals" (and yes, there were also lions in Europe back then), and while obviously feared it was nonetheless respected, much like Native Americans have that soft spot for bears in their mythology. There's plenty of evidence that bears were connected to some old deities (namely nature goddesses), and hunting bears was a rite of passage. Obviously all this symbolism is attached to the brown bear (Ursus arctos), but by extention the polar bear (Ursus maritimus) must have had a similar respect; after all, "Arctos" (greek for bear) was commonly associated with the north, which back then the greeks thought of as Hyperboree, a mythical marvelous land where the sun never set (of course, thats only in the summer...); by extension, we have those famous constellations named after the bear (Ursa Major, Ursa Minor, both after the latin "Ursus", meaning "bear"), and "Antartida" comes from "Antarctos", meaning "anti-bear" (after all, the south is the opposite of the north). Gradually, the ancestral connection with the bear degenerated; the Roman Empire thought of the once majestic animal as just the perfect thing to include in their cruel games, alongside many other species, that eventually disappeared locally from Europe and North Africa. The rise of Christianity only made things much, much worse; the majestic title of the bear passed to the lion, an animal with biblical connections and associated with Christ (you really thought C. S. Lewis made Aslan a lion at random?), and the bear became nothing but a forest demon, a monster that lurked in the woods. Following the extremely disrespect of medieval Christians for nature, out of fear for the Pagan gods, as well as the need to expand, the bear and many other european megafauna slowly declined. Nowdays, the brown bear only occurs in the northern and eastern Europe, with a very small population on the Pyrenees, which will die out very soon since the last female was killed. The polar bear escaped most of the persecution thanks to its northern distribution, but the conversion of the inuit into Christianity pretty much destroyed the old respect for the beast. Thus, while C. S. Lewis had forsaken the bear out of its demonic connections (curiously there's a few good bears in Narnia) in favor of the lion, Pullman used them. Indeed, refferences to the traditional Christian perception of bears is occasionally seen in the books; much disrespect to the panserbjørne is seen from the locals, while the king Iofur/Ragnar wishes to be human, perhaps seeing his own species as "inferior" because of the prejudice, much like afro-americans in the early 20th century. Worth of noting is how the bears were reffered as "demons" in the third book, and how in the first book Iofur/Ragnar considers that, to be a human, he has to be christian ( alongside having a daemon, of course)
In terms of their nature, the armoured bears are very interesting. While retaining sentience, they still behave much like their non-sapient counterparts: they are pretty much solitary predators, feeding on aquatic mammals like seals. Because they seem to be restricted, or at least concentrated, on Svalbard (an archipelago in the Artic Ocean; a very small range for a sentience species indeed), it seems unlikely that they are (very) territorial, and they seem to be flexible in terms of social behaviour, though as obvious they respect the ages old traditions; like real bears they don't form families, having the mother as the sole raiser of the cubs, but they are clearly seen to be able to feel affection as Iorek clearly shows. They lack a faith, and while they have a soul (their armour; presumably Dust connects with it, much like the "wheels" of the mulefas, and a story similar to that of the first mulefa or to that of Adam and Eve, starting perhaps with a snake and a female bear, was probably what led them to sentience), they supposedly lack a ghost, which frees them from The World of The Dead; in this respect, they are superior to mankind, being totally unaffected by the angels, and its perhaps not hard to speculate that Regent Metatron would have them killed for this. One thing that is quite important about the panserbjørne is how they aren't completly amoral even without religion, following an honor code that should be respected. As mentioned before, their solitary nature pretty much expresses their personal philosophy; they don't feel "cold", because loneliness is described as being "cold", and since polar bears are adapted to the polar realms this is very obvious.
So what to make of this all? To summarise, the panserbjørne are the opposite of the Christian animals of Narnia. While anthropomorphic, they are not human, but instead have a very different mentality. While able to feel affection, they don't mind being alone, which expresses independence; combined with their lack of faith, this means they are independent from God. Being independent from God also means that they are free. Coupled with all the anti-Christian symbolism of the bear, this means that the panserbjørne could be, to a certain extent, a model to follow. Nonetheless, much like people, they have their own share of bad things, namely how difficult progress is for them (then again, their lack of need for materialism could be a good thing). All in all, the ice bears are a magnificient product of the author's creativity, a very original take on the Talking Animal concept, and express that, while real bears are no where as cute as Disney portrays them, the trope Bears Are Bad News is not necessarily true.