It's not what I thought it was... Western Dressage, that is. I admit that I struggled with it ever since I joined WDAA. Now that I've read the WDAA or USEF WD rules (http://westerndressageassociation.org/wp-content/uploads/2011/05/WDRulesAllBreeds.pdf), I believe (competition) western dressage is dressage under western tack (just not including a rising trot) from my western perspective that is. Please read the rules and decide for yourself.
I like Al Dunning's version of Western Dressage: "A set of dressage-based exercises that result in a better broke horse". (Western Dressage DVD).
I believe a western horse needs to develop a rounded topline, lightness in the shoulders (and therefore a lower head to balance themselves) to be able to perform the traditional western maneuvers that we do with or without speed and changes of direction while performing them collected and in self carriage with no control (via a draped rein). I believe western riders need to learn to cue or communicate these requests eventually one-handed and with no contact on the reins - sometimes even bridleless! (I saw a bridleless western pleasure class on youtube the other day. Cool!) I don't see that reflected in the USEF WD rules. In fact, it specifies high head carriage, constant light rein contact and no draped rein.
So its still about teaching a western horse and western rider the skills necessary to do well in traditional western events... skills such as lightness, softness, suppleness, cadence, balance, flexibility and strength. To me, reining or reined cow horse is the highest expression of this ability and skill. As you can see the western tradition has always done it - we just didn't call it dressage.
IPHDA describes it well in the print below:
It's been quite a journey.
If you need more information, please ask. I'm most happy to chat.
Putting my spin on western performance dressage or body control.