i can think of any tough or leadership qualities related to being boneless.
Why Ivar was called the "boneless" is not really known, as we know this name only from the sagas and they are rather flowery in their language. From what I gather there are several interpretations of the name:
Supposedly Ivar suffered from brittle-bone-disease - this theory is based on the fact that in the sagas it is being mentioned that Ivar was carried on a shield by his warriors. A rather obscure expansion on this theory claims that "bone" is a mistranslation and refers to the Germanic word for "leg" ("the boneless" being a translation of "hinn beinlausi" and "Bein" being "leg" in modern German for example) and "boneless" rather meant "legless".
Yet another theory claims that "boneless" may refer to a high physical flexibility - Ivar being not only a great leader but also a great warrior and his skills made onlookers think that he had no bones at all, a claim that is also made of him in poems. This would also fit with the aforementioned being borne on a shield, as this was sometimes customary for victorious chieftains after battle (which of course could also have given rise to the name in itself, as the nickname "legless" (see above) could be attributed to him because of his many victories that left him being carried of the battlefield in triumph).*
Rory McTurk claims that in old Norwegian, "without bones" is a metaphor for the wind and Ivar being called "boneless" might be a hint to his mastership of the wind, that is his navigational abilities.
Sources:
Seedorf, Knud Stakemann: "Osteogenesis imperfecta. A study of clinical features and heredity based on 55 Danish families comprising 180 affected members", Copenhagen 1949
McTurk, Rory: "Studies in „Ragnars saga loðbrókar“ and Its Major Scandinavian Analogues", Oxford 1991
The second theory is very, very fuzzy in it's sources!. It's often only attributed to the "Háttalykill inn forni", an old norse poem. But since I can not find any translation of this online to back up the fact, I would mark this theory as highly speculative - just as the others, just with the added benefit of it being an internet narrative.