Although scattered to the ice ball of darkness and death that is Inwit, Dorn in due time came to be more closely associated with Holy Terra over time. Of his time on Inwit, little is known for certain. But this world of harshness certainly shaped him and he became as stone (proverbially). In time the House of Dorn of the Ice Caste rose to power and eventually claimed the entirety of the Inwit cluster. This, of course, helped the Great Crusade as the Emperor discovered not just his lost son, but an entire system of planets ruled by a space faring human empire with a wealth of barbaric human stock that were entirely suitable for upgrading in to Imperial Fists space marines.
Of course, Dorn himself is probably better known as the Praetorian of Terra. Far removed from Horus, he was not contacted by the Warmaster about the upcoming Heresy since to do so would mean too many risks. He was simply too close to the Emperor. And this simple fact meant that Dorn would become a loyalist for all time (not that he ever thought about Heresy since he was the incarnation of duty and loyalty to the ideals of the Great Crusade, but he certainly worried that whatever flaw was in Horus could also be in him). He's also well known for his fortress building and smashing (and his rivalry with Perturabo).
The first thing to note is that Dorn is a mild force multiplier. To be clear, he's not in the same league as Alpharius, but he does grant significant bonuses to his sons. The first is that he gives +d3 to combat resolution. This is huge and begs to be used in concert with a force that gets stuck in to close combat early and often. His sons can also use his Ld for pinning and morale tests, which is nice. And when he joins a unit, they benefit from furious charge and a few other rules that are clearly inspired by their Father's presence and zeal. As such I think these rules are very fitting for a personality noted for his fiery zeal in warfare and stone-like discipline.
He can slice his attacks value by 2 in order to gain S+2 with instant death. Most of this time this might not be needed, but his weapon (Storm's Teeth - a colossal AP2 chainsword) can benefit from this against enemy characters like praetors and equivalent.
Significantly, he can also provide a re roll of cover saves of "1" and pinning tests in up to 3 fortifications or terrain features anywhere that ordinarily grant a cover save. This is terrific. Not only can it be used defensively in concert with a directly purchased fortification (you must have the attitude of defence / come and get me, if you choose this), but could also be used to select mid-points of the battle field for the Imperial Fists to advance up to and in-between. This is terrific for a fast moving Imperial Fists build.
His armour is also interesting from the view that no-one can wound him on better than a 3+ regardless of source (but Destroyer weapons don't care about this, naturally).
Other than this, his weapon is bolted-like, but fires more shots and he can make terminators and phalanx squads in to troops. With the above rules, I think this urges the Imperial Fists player to build offensively with troops that move up through the battle field between terrain features and structures, trying to engage in combat with terminators and phalanx squads, whilst other troops (tactical squads) lay down withering fire from their BS5 bolters, coupled with support from heavier elements.
Finally, in 3000+ points games, he can select the Aetos Dios -- a unique thunder hawk gunship with a void shield. Pricey though in comparison to Dorn, even if it does not take up a Lord of War slot. As per my comments below, I could not imagine taking both Dorn and his gunship outside of about 4000+ points, and hence an apocalypse level game.
To be honest, Dorn is a rather cheap Primarch. He is not going to stand up well to the more close combat primarchs most of the time, even with his armour. Sure, he'll hold his own for a while, but the more rage-ful Primarchs will dispatch him eventually. Hence we are taking Dorn for his multiplier effects in addition to being a great warlord. He needs to watch out for Destroyer weapons as well as other more (ahem) "choppy" Primarchs.
His other equipment is not much to write home about, but at least he comes with a teleport homer (further encouraging a more terminator based style of play and army build, if any further encouragement were needed).
A solid Primarch, a cheap Primarch, and one to watch out for, but not to be feared too much by certain brothers of his.