I switched to Nikon about 8 months ago and I'm still loving it. I left for the image quality... and I haven't been disappointed by my D600.
One thing that I didn't expect to love? The lenses. I had always heard about the legendary Canon lenses... and I really thought that I would miss them (especially my 70-200 f/4 IS L)... but that's not what happened.
I bought the D600 with the 50mm f/1.8... that is a SPECTACULAR lens. So sharp. So fast and accurate to focus. Just beautiful bokeh - and CHEAP!
From there I sprang for my workhorse lens: 24-70 f/2.8G... sharp from corner to corner... unbelievably fast to focus... and incredible color rendition.
Lately - I just picked up a refurbished 70-300 for $350. I can't believe how great that lens is (especially for the price!). I wasn't expecting the AF to be up to tracking subjects... but actually it does just as well (sometimes better) as my 70-200 f/4 did with my 7D! Sure, it's not as sharp as my 70-200, but the IQ of the D600 more than makes up for it.
So? What do I miss about Canon? CANONRUMORS! ;-)
Seriously - I've just not been able to find another community that is as good as CanonRumors. NikonRumors is ok for rumors (although it doesn't get updated often enough) but the community is not great... quite a lot more amateurs and certainly not the great technical exchange that happens here on CR.
So if you're thinking of jumping ship... do it for the IQ and don't look back (other than back to CR ;-)
I'm in same boat, and actually went to Nikon for D800's DR and IQ, and for their UWA lenses -- of which I own the 16-35VR. Coming from the 5d2, 17-40L, 35L, and a few others (100mm Macro non-IS) I knew that all those would be covered. But seriously, I've never heard Canon had BETTER lens offerings...simply just more of a choice. That's drastically changed over the past 4-5 years with all of the new 1.8AF primes and 4.0VR zooms. I love both companies, and I have lots of nostalgic memories with my 40d, Rebel XS, and 5d2. Also, my 35L was a stunner. I loved that glass...wow, the colors and character. Even the build was beautiful. Each company has its own flavor and I agree -- CanonRumors is the best un-official corporate company website. Lots more bickering and negativity at times -- which can be blamed on 2-3X the number of members as other boards.
Don't trash Nikon, Sony, Leica, etc... they all have great offerings. Peace out. And BTW, the 70-300VR is a wonderful lens!! I have the 50 & 85/1.8G as well and they are phenoms. The 35L is still better than the 35/1.4G