Some random bullet points, mostly for friends of mine so I can point them at this as a War Steed introduction.

• You get access to War Steeds by doing the Epic quest in Riders of Rohan.  Volume III, Book 8 to be precise.  You can work through that Epic line as soon as you have access to Rohan, it’s well worth doing, because it has some of the best Epic ‘story’ quests since Moria.
• You seem to only own a single steed.  It shows up as a mount in the regular Skill | Mount window, near the bottom.
• When you first get the steed, it’s a Medium war steed.  There are three types, Light, Medium and Heavy.  You are not restricted to any one kind regardless of class, but some might suit certain classes better.
• The factor which determines whether you have a Light, Medium or Heavy is which Mounted Combat Traits you spend points in.  There are three lines, Light, Medium and Heavy and as soon as you spend your first point, your steed converts to a Light, Medium or Heavy steed (medium seems to be the default).
• You can reset the points for a few silver, so it’s not a permanent thing, you can switch between different kinds of steeds for different kinds of situations.
• There’s a new legendary item, Bridles.  Bridles come in three flavours, light, medium and heavy.  You can equip one at a time, they apply their benefits regardless of the type of mount (so a Light Bridle works even if you’re on a Heavy Steed), but obviously, because the traits vary, and the items affect the traits, a Light Bridle will offer more benefit on a Light Steed.
• Riding the horses is much tougher than travel horses at the start, so they won’t replace them straight away (maybe ever).
• Your steed has armour and morale, and damage is absorbed by the steed during combat protecting you to some extent.
• All classes get a new set of mounted combat skills (must be purchased), you don’t fight using your own regular class skills.
• Steeds earn experience and level up, allowing you to spend more points on traits.
• Quests offer XP, Legendary Item XP and now Steed XP.
• You appear to earn Steed XP from kills even while not mounted, but at a reduced rate.
• Mounted, Steed XP is equal to Item XP from kills, unmounted it appears to be worth around 1/3 that of item XP per kill.
• Steed XP appears to be earned on all kills, not just quest related kills.