LC's above average stats and very efficient signature card are more than enough to cement her into a top tier hero. Legion Commander's main selling point is her signature, Duel, further assisting Red decks seeking to utterly dominate the board.
Legion Commander's stats are surprisingly "middling" amongst red heroes (which still means they are great compared to non red ones). 8 Health is second lowest among her peers, only higher than Mazzie's 6. LC's attack value of 6 is average for Red, but 2 Retaliate from her passive essentially means 8 attack versus unarmored targets.
Moment of Courage plainly gives Retaliate +2. Retaliate does trigger off her signature, Duel, which is nice.
The most basic synergy of Retaliate is how it interacts with Negative Armor. Since it's a separate instance of damage, Retaliate 2 will do 3 damage to a hero with -1 armor. So Legion Commander will do a total of 10 damage to a unit with -1 Armor.
Ah, Duel. A simple, cheap, but extremely powerful effect. Having a 'precombat combat phase' between two heroes is pretty abused for 2 mana. The only 'downside' is it's limited to your red heroes...
Red is known to have the most powerful heroes on average, and the most combat oriented modifications. It's not unnatural to assume each time you cast Duel - the results will be in your favor.
Remember that you do not have to target heroes! Pushing out creep blockers for lethal or removing utility creatures are fantastic uses of Duel as well.
Legion helps any deck dominate the board EFFICIENTLY with Duel, and more than carry her weight in the combat phase. She fits into any deck with red, simply because red WANTS to have a strong presence since their spells aren't powerful in general. So if you're in red, LC automatically becomes a consideration.
Hero killer decks are a shoe in for Legion. No matter how your opponent tries to hide or protect their heroes, Legion forces them to come out and play for a staggeringly cheap cost of 2 mana.
The only deckbuilding limitation Legion Commander tags you with is the fact Duel can only target red heroes, so it's much more consistent the more red heroes you have.