I came to NATRC as an AERC LD rider.  I know the AERC rules – there aren’t very many and they’re pretty common-sense, straightforward types.  I knew NATRC had more.  I still find myself kind of fuzzy on a lot of them.  Some of the most major differences I noted:

NATRC: no help allowed.  Rider is the only one doing stuff with the horse from the time you’re vetted in to the time you vet out (or maybe a bit before/after, I was a little confused on exactly how this was supposed to work – but not having crew, wasn’t worried about it).
AERC: help allowed and even encouraged!  Others can hold your horse, groom, tack up, whatever.

NATRC: forwards motion must be on horseback.  No walking with the horse, not even a half-mile down the trail to find a better log to use to get on.  Walking back down the trail the opposite direction is fine, though, you just can’t proceed forwards.  Also applies to P&Rs/vet checks, where you have to ride in.
AERC: forwards motion is forwards motion, on or off the horse.  Totally okay to walk/run with your horse, tail up, jog down, whatever.  Often used to try to make it easier on the horse, sometimes used to make it easier on the rider, sometimes just a nice option to have.
I understand why NATRC has the rule, but this is still the one most frustrating to me coming from endurance.  I like being able to get off and walk from time to time.. down big hills, into checks, when I’m not sure I have her brain, etc.  It’s a little intimidating to know I don’t have that choice.

NATRC: minimum AND maximum time (roughly 30 minute window to aim for). Goal is pacing your horse slowly, not racing in.
AERC: minimum time.  To finish is to win, but indeed you could race if so desired.

NATRC: scored for horse & horsemanship & camp set-up – and frankly I’m not even sure what else, but I know there was more.
AERC: no points, just pass/fail criteria at the vet checks based on how your horse is doing.  Either you’re doing well enough to continue, or you’re not.

NATRC P&R stop: stand still for 10 minutes at the check, then have pulse & respiration taken. 16 (64) pulse OK, 17 (68) and you need to wait 10min and try again.  There are also points having to do with whether your P&R stays the same/better/worse all day, so folks may ask for a recheck if you think it should have been different.
AERC vet check: pulse taken as soon as you’re ready when you come into the check. Vet check follows and looks at the whole horse.  At least in my experience out here, 60 is fine, anything higher is not. Recheck whenever you’re ready.

That covers the basic differences that really stood out to me, but it didn’t exactly end there, either.
[Side note: a lot of folks that commented on Funder’s post seemed anti-NATRC.  I’d love to know why and what everyone else has seen!]

NATRC has three divisions.  For this particular ride, Novice and Competitive Pleasure were essentially the same from what I could see, same ~22(?) mile trail with a speed of 3.5-4mph.  Open – what I rode in – had ~26 miles of trail and a speed of 4-4.5mph.

I know there were people in the N/CP division who stayed at a walk the entire time.  They may all have been on gaited horses, but that just struck me as appallingly boring.  People didn’t want to ride Open for various reasons – too fast, too hard, horse not conditioned enough, etc.

Open riders were praised effusively at awards for doing such a long distance and such a fast speed.  Mount Diablo is supposed to be a particularly difficult ride to make time on.  People chose not to ride Open because they felt their horse wasn’t well-conditioned enough.  For Confetti and I, Open was not a challenge.  I feel like we could have done it at a 5mph pace – or at least gotten close to it! – without even getting close to running out of horse.  It never felt like she was really working, though. She was more willing to walk towards the end, but she never got tired or thought about quitting. I held her back almost the entire ride.

I rode the horse I had at the ride. I sat the trot up and down the hills to discourage adding speed. I hauled myself and camelbak up onto the horse – not much grace, but she tolerated it and we generally find bigger hills or stumps at home. I ground-mounted during the ride where needed.  We worked through some herdboundness and never totally lost her mind.  As a training exercise, it worked pretty well for us.

If I had really been training for those obstacles* and all the judging and stuff, I’d be disappointed at not getting anything. AERC has made me a convert, though – I finished the ride with a happy, sound, energetic horse who would have done another loop if I asked.

We had four obstacles, if memory serves – I haven’t gotten my scorecard back yet since I opted not to stay later in the evening.
– judged mount first thing in the morning
– navigate off-trail briefly following the ribbons
– sidepass to a tree, pin ribbon, sidepass away
– big trot/canter on a loose rein, halt, back up, trot off

Would I do it again?
Short answer: maybe.

At this point, I have no desire to ride a NATRC ride just to ride a NATRC ride.  Confetti and I find joy in doing a brisk trot down the trail, flying up hills, jogging together down particularly steep sections.  I’m not interested in chasing points and doing things a particular way just because some idealized rule says they should work that way.
 
I don’t regret going, not at all.  It was a great training ride for us.  Everyone was friendly and helpful (even though I was the last one into camp and I think the first to leave!), willing to share advice, answer questions, etc.

I think NATRC Open rides would be a great sort of intro-ride for folks looking to get into LDs but who aren’t sure about the speed & distance combined.  If you can get through 27 miles at a tiny trot/mostly walking 4mph?  5mph may not seem so bad.  I’ve mentioned to several friends that I would happily go with them to this ride next year.

If or when Confetti deigns LDs to be too much/too difficult, I will probably consider taking her to NATRC rides as ‘retirement’ from AERC.  Slower rides, shorter distance, keeps us with a goal to work towards. 

I can appreciate where NATRC is coming from with their bunches of rules and scoring even as it’s not something I aspire to currently.  No hate for NATRC from me, it’s just not my thing!