From what I have read, the reasoning behind the lack of diesel Power Wagon has at least 4 points:
1. the intercooler and winch want to occupy the same space
2. the diesel is considered (by the designers) to be too heavy to make for good offroad performance (regardless of what arguments can be made as to the pros and cons of diesels for offroad, on solely the issue of front to rear weight distribution this is undeniable)
3. the longevity of the current frontend parts (especially balljoints) under a diesel used offroad (to the high extent that they intend the Power Wagon to be used offroad) would likely be alarmingly short
4. The rearend used in the Power Wagon is the 10.5" AAM axle, same as a Hemi Ram, but with AAM 11.5" axle shafts (not so for the new 2014s as they will utilize the AAM 11.5") the ring and pinion is not viewed as being up to the task of handling the Cummins' output.
Power Wagon desirability? Part of it stems from rarity, they keep production numbers fairly low thus lending a bit of exclusivity to ownership (some dealers don't even know what a Power Wagon is!) but, rarity isn't everything- as I've heard said before- "It's rare that I poop my pants but, that doesn't make it desirable!" The real desirability comes from:
approximately 2" lift
more flexible springs for greater articulation
Power Wagon specific frame part number (specified in order to facilitate the mounting of a LOT more skidplates than a standard 2500)
4.56 gears and locking differentials front and rear (also, rear acts as a limited slip when not locked, and rear has AAM 11.5" axleshafts)
Electronic antiswaybar disconnect
12,000lb Warn winch (which, if you look at the power rating, gearing, and cable size actually appears to be a short drum, reverse rotation variant of a Warn 15,000lb winch)
Larger more aggressive standard tires (285/70R17 BFG all terrains as opposed to the smaller all season tires (245 section, IIRC) that come on a standard 2500)
Cool stickers (ST/Tradesman, SLT package PW's)
Cool letters (Laramie PW's)
Now as to why the 2012 in particular would be desirable over other years (I am only addressing 4th gen years)? I'll admit I am at a loss on that one! Although I have a 2012, I'd almost prefer a 2011 since they don't have traction control (it can be a bit intrusive at times) but, I guess if someone likes the idea of having traction control, that would make a 2012 better than a 2010 or a 2011.
Versus a 2013, it would be entirely subjective to state that the 2012 would have any advantage. Many like the 2013's new look better. I am actually one of the few that vastly prefers the 2012 over the 2013. I do not like the new interior, grille, headlights, or badges that were added for the 2013 model year BUT, that's just my opinion. DON'T get me wrong, a 2013 Power Wagon is still an awesome truck!!! I just have my (admittedly biased) preferences.
Versus a 2014, I would hands down take a 2012 or 2013. Yes, the 2014 gets the new 6.4L (the engine I think should have been in the Power Wagon ever since the 2010!!! (first year of the 4th gen which is longer and heavier than the 3rd gen PW)) I am VERY VERY VERY jealous of the 6.4L!!! But, the 2014 comes with two features that I feel are disadvantageous:
rear coil springs
front axle disconnect
Why would I say rear axle coil springs are disadvantageous? Don't they offer a nicer ride? Can't they be tweaked to offer tons of travel? Aren't coil springs the way of the future anyway? Well.... yes, they do offer a nicer stock ride BUT, they will get into BIG issues when we start to modify these trucks:
obviously suspension manufacturers will have to make new kits so, there's going to be a bit of a wait
once they do come out, the new lift kits are going to be big $$$$ to maintain proper geometry and add articulation and lift
extreme articulation is going to be difficult due to the small spring height possible when trying to keep the coils below the bed floor
long arms (to maintain proper geometry with longer travel) are going to be hard to package what with the fuel tank's location (imagine trying to stuff a triangulated four link under there!!!)
But, what about front axle disconnect? Doesn't it add approximately 1mpg by eliminating some of the drag produced by the entirety of the front axle parts rotating constantly the way they currently do? Yes, it does add about 1mpg but, at what cost? Anybody remember the front axle weakness brought on by central axle disconnect in the Jeep Dana 30s, and even in Dodge Dana 60s of years ago? Another maker's example- anybody remember having a 4wd failure due to GM's famous "donkey dick" (pardon the vernacular but, that is the less than affectionate nickname for the part) vacuum actuator for the front axle in four wheel drive? Over the years, axle disconnect devices have repeatedly proven themselves as less than desirable. It's too bad that we now have to have these because of a combination of two things:
C.A.F.E. standards (good thing in many ways, bad in many ways too) and the fact that we are too coddled in our cushy trucks to get out and lock the hubs! (I wish manual hubs were going to be an option instead of the disconnect, I would definitely check that box!)
To me, the 6.4L is a HUGE advantage to the 2014 BUT, the rear suspension and the front axle disconnect are too much of a disadvantage to overlook. (to enough of an extent that I was contemplating trading my 2012 in on a 2014 (to get the 6.4L!) but decided not to partially because of the rear suspension but, mostly because of the front axle disconnect) Thus, the 2010-2013s are the most desirable 4th gen! But, that's just my opinion!
BUT, to me the MOST desirable Power Wagon is a 3rd gen, regular cab, six speed manual!!! (good luck finding one!)