For me, it was largely my wife's fault. When we met, she had a konica P&S for film. I accidentally put it in the washing machine (long story), and it was no more. Next up, she got a IXUS APS and liked it. Then she got a EOS 500N from her father, which we used for a loong time.
When digital turned up, we started from scratch, as we wanted first a P&S. We looked at all the brands, but my wife had two criterias: IQ, and skintones. She disregarded all other brands after looking at pics, and we became the owners of an A80, which we loved. Adored the vari-angle screen.
A year later, it falls out of our car, and we get an A95 on the insurance (same, but better than A80).
Three years later, we get married. (yes, she is my wife now, but we were together for many years before it happened). So, what to give my wife as a "morning gift"? (old tradition in my country).
The 40D had just come out, but I got a sweet deal on the 30D, and funds wer limited, so I went for it (thinking I can use the old sigma lenses for the 500N for it). The 70-300 worked, but the 28-80 did not, so I sold that package off to a relative, and bought the Tamron 17-50 non-VC.
And now? Several lenses later and flash etc.? I will not switch brands unless Canon goes bankrupt or starts using baby hearts to power their cameras (or something similarly unprobable royal screw-up).
Ergonomics. Nikons feel weird.
Total package. Room to grow that only Nikon can compete with.
Colors and skintones. (yes I know you can change it, but I don't know how, and with Canon, I don't need to...)
Lenses. Both Third-Party and the lovely USM ones
Build quality. (thats the reason I chose a 30D over a Rebel, it has been banged up many times, and still works as new, over 120.000 actuations later)
Well...price? (but you DO get what you pay for, both in houses and lenses)
DR does not affect me that much, as I am not remotely pushing the boundaries there. I want fast autofocus and good Hi-Iso performance.