My desire for wanting to go full frame is simply in lenses. I grew up shooting on film, both point and shoots and on SLR's of just about every level. To me, 24mm is 24mm. I know that it's going to give me an 86* Angle of view. To get a near equivalent on a crop body, I'd need a 15mm lens, which gives an 86* angle of view. But I need to be realistic here. I love shooting wide angle for my own interests outside of my portrait studio. Though I don't have one, the widest (zoom) lens Nikon makes starts at 10mm, which on a DX gives 109*. Wide, yes, I'll give it that. But I don't like how slow that aperture is (f/4), hence I ended up with a Tokina f/2.8 11-16, which starts at 104*. But even if Nikon made a comparable lens to, say a 14-24, it would have to be somewhere in the neiborhood of 8 or 9mm - that 15mm I mentioned earlier. Now of course, I could sell my 11-16 and replace it with a Sigma 8-16, but again...there's that nasty aperture again, which is variable at f/4.5 at its widest to f/5.6 at the long end. It's not worth it to me, despite the fact that I rarely shoot faster than f/8 for landscapes. Though, I will admit, 2.8 does have a good deal of advantage when shooting hand held at dusk, which is why I mostly use fast zooms. Yes, for the most part, working that fast just isn't needed in most situations, but I live in Alaska where fighting available light, come winter, is what we do here. Even on a tripod, sometimes I struggle to get the shot I'm after, as I have to keep the shutter open longer.
That's all wide angle though. When I think about the longer side of things and shooting, say birds or distant wildlife, You bet, even if I currently had a full frame, I'd be reaching for my D7100...but I always use FF long lenses. at 300mm on a crop sensor, I'm looking at roughly a 5* angle of view. To achieve that same angle on a full frame, I'd be looking at needing a 460mm lens, which, as we all know gets pretty expensive, and I don't use teleconverters unless I have no other choice.
Realistically, there are pros and cons to every format. DX (or APS-C) is really good to make use of longer lenses for when you need that extra reach. But when you need to go wide, unless you're going to a mirrorless system, there really isn't a better alternative, especially when you take lenses into account. But I need to be honest here. M 4/3 puts us back to square 1, in terms of angle of view/mm. Its directly the same as its full frame counter part, when you divide its focal length in half. My problem with with M4/3 is purely in its focus tracking. Though it's gotten better in this last year, it's still not quite up to where I'd want it at, personally. They produce some outstanding images. I'll give them that, especially when you're looking at the EM-1 with the more expensive lenses.
There's a tool for every job. Sometimes a full frame camera is the answer. Sometimes, it isn't. As Solarflare points out, the best glass is in the realm of full frame, though if you look hard enough, some crop sensor lenses can come close. I know I want to eventually get a full frame camera for reasons I mentioned above, but would I use it in my portrait studio? Maybe, maybe not. And you just can't beat the reach of a DX body with full frame lenses in terms of going long. Realistically, the average person wouldn't be able to tell the difference in an identical images, shot with a crop sensor and a full frame. You really have to know what you're looking for. When it comes to taking a hike up a mountain, would I want to carry even my D7100 and a couple lenses? I can tell you now, I wouldn't...It may not seem like much when you're packing up a bag at first, but that body and a couple-few lenses gets heavy after a while. Which is where a nice mirrorless system comes in...
And what would be consider "full frame" in film photography ? 35mm, 6x6, 6x9, 4x5, 8x10, 12x16 or 20x24 ? Or maybe "life size" ? http://graphics8.nytimes.com/images/2011/08/17/t-magazine/17kase-sayej/17kase-sayej-tmagArticle.jpg
(Just for fun !)
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