I have been a long time Nikon shooter who switched to Canon a few years back. I am thinking of returning and here is why....
1) Lenses. As a prime shooter I find that Nikon's prosumer line-up of primes is better. The 35 1.8 DX, the 50 1.8 G, and now the recently announced 85 1.8 G. It's the holy trinity of affordable primes. To me the aging primes in Canon's line-up leaves a lot to be desired.
2) Bodies. I miss Nikon bodies. They just feel so much better in your hands and there is nothing like the connection that one has with his/her camera. That being said I do find the placement of buttons and menus more logical on Canon systems.
3) Focusing. Nikon's focusing: one word accurate. I have owned a number of bodies over the years and even Nikon's cheapest SLRs are spot on accurate. I rarely ever had an OOF shot with a Nikon SLR. With Canon I would say my keeper rate is 1 in 3.
In Canon's favor I prefer the Canon 'look' to it's image processing (and this does count for a lot when making my decision). I often here people state "shoot RAW and color doesn't matter argument", but that is a bunch of b.s: I find Nikon and Canon cameras even using Lightroom deliver different looks - it's a signature of the camera itself.
I definitely have trouble with this decision, I find both systems have an equal share of pros and cons. Sometimes I wish they would merge LOL !