I've been having a lot of fun and really love this class. There's a few points I wanted to make in terms of feedback for how it feels to play and this is primarily from a PvE perspective of playing about on Arelith.
Iron Stance feels 100% necessary however once you have it, there's no way to turn it off when you have situations where you'd rather have a middle ground between movement speed and slight defensive bonus. Having the Iron Stance feat as something which toggles which defensive stance you're activating would be really helpful here. It's mostly just to give the player the choice of which one to activate, I would recommend not letting players toggle between both stances whilst it is active but rather toggle during downtime.
Debilitating Strike I, II, III sucks to use. There's definitely a really niche situation where you'd want a STR based character to have it in order to try land movement penalties, but it's a 3 feat investment (one of those is an epic bonus feat) in order to do something with limited range and to a similar effect as Dirty Fighting. It is not an instant attack either. If it became a single feat (taken as epic bonus feat) and just did the current ability, the investment would make more sense.
Banner is fun to use but due to it taking a round to put down, you lose a bunch of the limited time that you have for it. It's very limited in range and cannot move, so to have to use up a round to put it down and then get a more limited use of it. If it lasted 2 minutes and then had a 2 minutes downtime, maybe this wouldn't be so bad. Or dropping the banner down as an instant action would be more useful. A player still wouldn't be able to move it around, so you're limited to fighting within that little area. We also have banner backpacks and VFX options that could be really cool to use. What if banner was used on the character for a reduced benefit which could move with the earthkin defender?
Mounting Resistance I, II, III rocks for a really comfy solo experience and mitigates damage as you're going. I found these to just be really useful whilst playing the character generally.
Deterrence not adding biteback damage for the character feels dumb. It is ONLY relevant in PvP because in PvE the Earthkin Defender is usually using -guard on their allies, so they're the one taking the brunt of the incoming attacks. It's a small amount of biteback and there are other sources already. It really should be applied to the defender.
I can see why a lot of the decisions were previously made about this class because it was developed during a different time. However, since then we've had some crazy powerful classes like Paladin and Harbinger that really change the way things like this play out. There are so many more instant actions in today's Arelith and as a result it's very hard to get an Earthkin Defender's stuff going in a relevant timeframe.