The 'Buff' Faction - There are so many elements in Trolls that need to be considered, and most of them are buffs to their models. Troll statlines are typically unimpressive for their costs, typically sporting average or lower-than-average damage boxes on their warbeasts, their defensive stats typically leave something to be desired, and their MAT/RAT values rarely exceed 6/5. That said, almost all of their warbeasts/solos/units have animi or abilities that improve range, movement speed, accuracy, damage rolls, DEF, or ARM. While it's impossible to include every option the faction has, the net effect is that each element contributes to the overall strength of your troops, allowing overlapping areas of strength. Some of the more well-known support pieces include the Kriel Stone, Fell Caller, Stone Scribe Chronicler, Impaler, and the Axer - but they are far from the only ones the faction has. In short, Trolls can be faster, hit harder, or be harder to kill than their statlines first imply.
Tough/Regeneration - I almost didn't include these two things since they are on every model in the faction, but it still bears mentioning. Having every unit being Tough means that your opponent will have a harder time removing any of your infantry from valuable positions. Sure, they could kill them, but unless they have some specific tools, they can't rely on it. Anyone who's played against Trolls knows how annoying Tough can be. Similarly, Regneration can cause headaches if you find yourself unable to kill that annoying Dire Troll that might cause you to lose the game next turn. While it might seem unimpressive, being able to spend only one fury and having a really good chance of having a fully-functional warbeast can make all of the difference in the world.
Medium Bases - The fact that most Trolls are on a medium-(or larger)-sized base can be both an advantage and a liability. Medium bases are much easier to target with enemy ranged/magical attacks, since they can effectively ignore their own infantry, meaning that your own support models and your warlock will be more vulnerable to attack than they might be otherwise. It's much easier to draw LOS to a bigger base and this is why many Trolls "brick up." On the other hand, medium-bases mean you're immune to Tramples which means your warlock/support models are less vulnerable to warjacks/warbeasts. In essence, Trolls can shut down warjacks and warbeasts and are vulnerable to ranged/magic.
Shifting Stones - While these guys are not nearly as scary as they were in Mk I, they are really effective at providing additional attack vectors for Circle warbeasts while also offering some limited fury management. If you can kill one of the stones, you'll be better off, but many make the mistake of thinking that two shifting stones aren't a threat, when they can still heal models (even constructs) and remove fury from warbeasts.
High Threat Ranges - This is a very general theme throughout Circle but one that needs to be mentioned. While the faction still doesn't have access to Rush (yet), Circle still has many ways of getting ridiculous threat ranges for their warbeasts. Hunter's Mark from the Blackclad, the aforementioned Force Bolt on Druids, Lightning Tendrills granting Reach, Dog Pile, Bounding, Warp: SPD on the Feral Warpwolf, numerous Pathfinder models and lots of effects that grant free charges means that Circle is going to be hitting you from really far away. The longest threat range I can think of is a Feral Warpwolf charging a target hit with Dog Pile and Hunter's Mark with the animus Bounding on him and warping for SPD for a whopping 17.5" threat range with eKaya. Yikes!
Legion of EverblightEyeless Sight - While Eyeless Sight took a huge hit in the recent errata (no more ignoring LOS), it still ignores concealment and Stealth so it still has amazing applications for ranged attacks - of which the faction has more than any other Hordes faction. Considering that many enemies rely on concealment (like forests) or Stealth (like Bane Thralls) to stay alive, Legion's shooting is as efficient as it is prolific. The added benefit here, of course, is that most Legion warbeasts have a ranged attack so that also means plenty of boosting damage rolls - something that Warmachine factions can't do nearly as often.
Numerous Warbeasts - Legion was always intended to have more warbeasts than any other faction and as such they have lots of fury-mitigating effects for their warlocks (Absylonia, Thagarosh) or models (Shepherd, Forsaken). This means that Legion forces typically contain 2+ heavies, maybe a light 'beast, and at least 1 Shredder (for Tenacity) or other lesser warbeasts. Also, Legion warlocks sport a higher average fury rating than other factions (except maybe Circle) so they'll be casting more spells and animi than other factions and have a lot of freedom in forcing their warbeasts more than other factions. Their warbeasts' statlines thus tend to seem underwhelming for their points cost but seeing as many can force for "free", they are much better than they look on paper.
Damage Type: Fire - Much like Menoth, there aren't many models with Immunity: Fire, but here it's less useful than it is for Menoth. Why? Legion has fewer Continuous Effect: Fire attacks than Menoth, and that means that models which have Immunity: Fire will be more resilient to the effects and damage of Legion's attacks. Since Legion has lots of ranged attacks as listed above, and many of those aren't Continuous attacks, those models who are immune to fire will be truly immune to your attacks. That means that those Druids with Elemental Immunity from their Overseer will be truly immune to many of your ranged attacks, where they might be otherwise killed by Menoth's Continuous Fire.
SkornePain Giver Beast Handlers - Every faction seems to have their own take on warbeast/warjack buffing/efficiency, and the Beast Handlers are how Skorne do it. These guys are probably most analagous to a Menoth Choir with some slightly different abilities and more potential damage output (although only slightly). These guys can Enrage a warbeast to give +2 STR and that means a huge improvement in damage output from Skorne warbeasts (especially the heavies). This takes the warbeasts from 'relatively uninspiring' to 'scary!'. These guys also offer some efficiency in the ability to add or remove fury from warbeasts (friendly or enemy) when they deal damage. Since they have Anatomical Precision, this means they can literally whip their 'beasts into shape and remove excess fury if you ran a little too hot. Unlike Menoth, however, Skorne warbeasts (the titans, especially) already seem pretty awesome without their beast handler support - sporting solid defensive and offensive capabilities for their price - compared to other warbeasts of similar cost. No 'beast handler tax' here!
"Slow" but powerful - Skorne is a slow-moving, death-dealing machine. Let me clarify. Many of the elements of a Skorne army will move fairly slowly towards their intended target and Skorne relies heavily on murdering the enemy in close-combat. That said, there are spells, animi, and abilities that allow a surprising amount of speed from a faction which seems to be much slower. Rush from the Gladiator is an obvious spell for increasing threat ranges, but with the lower-SPD on many of their warbeasts, this animus is mostly used to allow your heavy warbeasts to merely keep up with your own models or just barely out-threaten some warjacks. There are exceptions, of course (Molik Kharn), but generally Skorne won't get to the party first, but they'll probably be the last to leave. The Bronzeback Titan, for example, might not get there first, but he has the potential to wreck a Colossal my himself with full fury and the support of some Beast Handlers. No other 9 point heavy warjack/warbeast will be able to do the same.
Limited Ranged Support - While it is clear that Privateer Press is seeking to change this a bit with recent releases, the fact remains that for what few ranged elements exist in Skorne, there is little support. In fact, perhaps the best "ranged" support spell is Black Spot with epic Hexeris, and even that spell is just as effective (if not moreso) on melee attacks. Indeed, even with the Cyclops Raider sporting a solid ranged weapon and the animus Far Strike, Skorne is a heavily melee-centric force - and they're damn good at it too!
So those are some things that need to be considered when looking at each faction so you're not looking at them "in a vacuum." I decided not to touch on Minions since there is so more that you need to consider with them since there are two separate sub-factions that use different models.