Lightning does the same damage as on foot. Except with HUGE DELAYS to build up fury or wait on cooldowns. On foot, I can do 3 or 4 hits that total more than the single full fury hit that I can do mounted in the same timeframe. If I spam mounted, the damage is much less due to lack of fury, its more than on foot for the initial burst but cooldowns prevent sustained dps for that method. And I run out of power mounted -- some annoying elite with 70k ran me out last night and that took forever, dismounting was not an option there were too many chasing me (to be fair I would not have won THAT on foot, mounted kiting is superior if slow).
Maybe it gets better. But if it takes a maxed out mount to be viable, turbine missed something--- people want to try it out right away, and having to get 15 levels to even be able to steer and another 25 levels to be able to do any damage gives folks a negative perspective of the steeds right away. I will keep trying it now and then but for now I am mostly using the steed as a faster horse and jumping off to fight anything except the bigger warbands.
Let me be clear, too. The RK is my main but I have a mounted burg and champ now. Both are far superior to the RK for damage. THe burg oneshots most mobs with his backstab, and mopping up is easy. The champ just smashes things with big hits. And the RK is higher level, has better gear, and a higher level and more tweaked mount.