I have it on my D70, really matches to the D70S except for LCD screen size. Menus are better, more easily navigable. It was free years ago. If it is cheap,go ahead, I like it. I still have my D70 and let my daughter use it.
I don't know any functional differences, certainly no drawbacks.
Looking online, it says there is an improvement in the AF functions and options for direct printing from camera are increased.

I don't think any answer actually hit what is different in a full frame.
You lose depth of field with a full frame (for out of focus backgrounds in a portrait say) so they achieve a look with fast glass not possible with a crop sensor.
Also for a given pixel count, noise is lower/dynamic range (mostly shadows) is higher because of larger photosite size.

Here's how with less touching the camera.
Set up your manual bracket, but meter for either highlights or shadows as your extreme exposure, three exposures takes you back to base, "0" meter.
Set your exposure compensation to what is appropriate, I might use -3 or -2 usually.
Take a manual bracket of exposures (+4, +2, 0) and then switch to Aperture Priority. Take a compensated bracket of exposures (+1, -1, -3).
You have to be carefull with the spot metering point you rest your composition on. But touch the camera once if you have a release and get that all done automatically.

You prefer the line of sight non-digital only can be used with a work-around to plain optical slave expensive as a modern solution 20 year old SU4 to a PocketWizard or QFlash?

You prefer the line of sight non-digital only can be used with a work-around to plain optical slave expensive as a modern solution 20 year old SU4 to a PocketWizard or QFlash?

I prefer the Pen Tool. Take time to learn to work the curves and points. Learn to make a selection while saving your pen path.
Learn about feathering a selection. Here is a picture I made removing the background, with a 2 pixel feather to make it look natural.

SU-4 is a bad idea.
Look at the radio remotes with hot shoes.
I've been thinking of some cheap ones from Adorama, $30-$40 a unit, non adjustable exposure.http://www.adorama.com/FPRRS4.html
The better ones like Pocket Wizard and Quantum can be adjusted on the fly just like iTTL.

I have several 600s and a D90. The Commander mode doesn't enter into the image, so no reason not to use it exposure wise.
Commander uses pre-flash to direct the remotes, then you can also have a flash in concert with the remotes that affects exposure. I often use a very low level on camera flash.
If it makes the subject blink, you can cover with a piece of unexposed but developed film, that allows the IR signal but blocks the visible light.

As for Nikon, they will not even repair for money if it is Gray Market.

If you use a rectilinear (non-fisheye) lens, anything parallel to the plane of focus will be straight.
So keep the camera level, aim at eye level subjects, zoom wider to capture image high or low and crop out extra floor later. Always shoot straight at a wall if you can't stand it tilting in.
Or get a shift lens, which performs the above operation for you.

Unless you are specific, there are winners and losers.
I use a D90, right now I have a Nikon 55-200VR (cheap and good), a Nikon 50 afd 1.8, and a Tokina 12-24 f4.
I have had all those brands and more, each lens is its own.
I really recommend the Nikon 18-55 and 55-200 both in VR if you don't have stronger opinions or more specific choices.
Also, look at KEH.com if your budget is tight. They are used but perfectly reputable with a return policy.

I would say more like being able to creatively use:
-freezing motion with a fast shutter speed (hummingbird wings)
-showing motion blur with a panning shot (clear detail in a speeding car with the background blurred)
-using off camera lights to control shadows (flattering portrait)
-elapsed time photography (use of a tripod and release, knowledge of cumulative exposure)
-capture a split second occurrence (bursting balloon or water drops)
-properly exposed night time or astro picture (calculated exposure)

I shoot Nikon, and I'm slowly looking at 85 1.4 and 105 f2DC.
For the money and effect, they are right that a Lensbaby will give the quickest boldest results. Different sort of look though.
Good luck

I have a D90 and the Nikon 55-200 VR. I wouldn't bother to take a chance on the Sigma, got my 55-200 for $150 refurbed. I bought it as a temporary replacement for my 80-200 2.8 Nikon, now it is my go to lens. Sharp as any down to pixel level if you do it right. Very light. It's just cheaply built. Doesn't have a big aperture. Pictures are as excellent as I can make them in real world. Very light and easy to use, compared to my weighs a ton 2.8. Give it a try.