Just my opinion, but it would work like this:
The mount needs to be at least one step larger than the rider. So a pony-sized (Medium) PC can't carry another medium PC, but if you can wildshape into a Large creature, it's fine. (If you're a pony, you can carry a halfling PC fine.) This is slightly different than the official rule, which requires the mount to be at least Large.
The rider and mount should coordinate initiative. Of course, they probably won't! (Not actually a rule.)
The rider no longer needs to spend a move action to move (since the mount is doing so, and it's the mount making those decisions), but unless the rider gets off they can't move independently. However, both the mount and rider get their own standard actions, on their initiative, which is not standard. The rider does get a move action, if they can use it for something other than movement (or use it to teleport or levitate off the mount, etc.) For some PCs, like a thief using Tactical Trick, this is probably less than ideal. For a slow-moving PC without any special movement powers, this is ideal.
If the rider charges... well, it doesn't work, since they're not controlling the mount. Only the mount can charge. There may be a way around this, involving spending feats, or probably a warlord power.
The wildshaped creature only gets whatever powers the wildshape power provides. I don't know if you'd be as fast as a horse, but probably not. Still, you might be fast enough to effectively increase your ally's speed. You won't get the Charger benefit or be able to trample unless you've got a wildshape power that specifically provides that.
The Mounted Combat feat is not required, and in fact can't be used to gain any benefits, since the wildshaped creature doesn't actually provide any. (The rider doesn't get to use their skills in place of the mounts, unlike with a regular mount.) So make sure the druid has skills like Athletics! However, if you don't have the feat, the mount takes a -2 penalty to hit, as normal. This is probably the biggest penalty to this tactic, but is admittedly no different than riding any sort of combative mount. (Not in the rules, but if the druid could take a similar feat to take away the penalty, I think that's fine too.)
The wildshaped mount and rider count as being adjacent (usually the case when you share a space). This can be an advantage, and a disadvantage. It's really handy if the rider is a defender of some sort, since they can protect their mount with Battle Guardian or what not.
Opportunity attacks are provoked as with a regular mount. (So an opponent only gets to target one of them when they provoke an opportunity attack through movement.) This can be handy.
The mounts in enclosed spaces rules does not apply, as the mount is actually a sentient creature. Squeezing rules still apply, since the mount is generally Large.
