My D70 has been the most frustrating camera I've ever used.
Mind you, I came to digital (aside from P&S) from shooting Tri-X and color neg in 120 and 4x5, so there was going to be some adjustment (as if I suddenly threw out everything and bought Kodachrome in bulk), but I'd shot enough chromes to have a feel for the narrower spectrum.
Most of my 120, 4x5 and 5x7 color work was with positives, so my transition wasn't nearly as painful, but I'd like to address a few of your points...
I have to make major changes to every single RAW or JPG (depending on what I'm shooting) - underexposure by one or two EV, white balance is never
If you're underexposing, switching brands isn't going to take care of that, like shooting positives, digital needs you to be as far right as possible, fortunately we get histograms for checking exposure. You should learn to evaluate your histograms as you start to shoot, or in pre-shoot mode with some test shots (like shooting 'roids on a 4x5 back.) You may also want to Google for the white towel exposure PDF someone has up, as it'll help you in nailing your exposure, or test (like you would a new batch of film) the sensor against the light meter you used for 4x5 and get dialed in.
Underexposing by 2 EV on a Canon won't make you happier.
correct, flash is basically a crapshoot (SB600 bounced, for the most part).
If you're having issues with flash, again switching brands won't help, Canon doesn't come close to Nikon in flash, after a couple of shots to dial in compensation, I've yet to have significant problems with my SB800, and I tend to WB later, as I shoot raw, but I don't shoot with a D70 all that often, and I don't use flash on that body. I've had the 800 on my D2x and a D200 I used to own, and WB was darned good, even in mixed lighting, but for strange mixes or criticality, use a card and get it in post as a batch adjustment.
Nikon's prime lens line (dollar for dollar in terms of aperture and focus speed, they're plenty sharp enough) pales in comparison to what I can get from Canon (and what I used prior to the D70).
I wouldn't say focus speed "pales," but price-wise Canon wins hands-down in the long glass. I've shot Canons in the same category from the same time period as the D70, and they're pretty equivalent. Newer bodies from both manufacturers are better in AF accuracy. Speed-wise I'm not sure that there's much difference in the second generation digital bodies- I don't notice a significant speed difference in focusing from my old S2 to my D2x, even though the D2x is way better powered (with AF-S lenses, with screwdriver focus lenses, there's a difference in speed.) I do notice an accuracy difference though.
If and when the successor to the 5D comes out, I'm running back to Canon and my 50/1.4 as fast as my credit card can take me.
Why wait? You're obviously unhappy- and you're missing pictures now. Personally, I don't think you'll be all that much happier switching brands until you get your exposure issues resolved. You'll get pictures off a depreciating body, and recoup some investment by reselling it. You're already two generations behind, and waiting isn't going to help you in the interim. If you're that unhappy with the D70 and Nikon, switch now- you're missing shots with a body you'll be happier with. Plus the resale value of the D70 at that point will be near zero.