It is fairly straightforward to qualify as a skilled worker if you have a doctorate, but because the idiots at the CIC dropped the points required in 2003 so low that almost anyone with a BA could qualify, you will be in for a wait for a couple of years before your visa gets approved.
From a tax standpoint as a US citizen you have to file a 1040 every year until you die, although once you are non-resident you do not have to pay US
taxes except on US-source income, and if you reside in Canada there's usually a way of claiming a foreign tax credit for it.
The big snag is that in 2006 the US capped the foreign tax credit for non-residents, in 2007 it was $87,500, so essentially if you earned more than that, you would be subject to dual taxation (although this may not be a problem if your income is mainly US-source). There are other deductions you can claim and the limit will have obviously gone up a fair bit by the time you immigrate, but life will get complicated for you if you earn over $100,000 a year I suspect.
From a tax standpoint it also gets complicated if you work in both countries. The 2008 tax treaty requires tax to be paid proportionally to both countries based on where the work was performed. This isn't terribly difficult if you have two employers (or one employer who has a business in each country) but if you're self-employed it's a bit of a nightmare from a tax standpoint because you have to do payroll withholding in two countries.
If you do all the work physically in the US and you simply live in Canada, you will pay US rates, then claim a foreign tax credit in Canada, then pay any difference on top of that (which there almost certainly will be), so you effectively pay the rate in the Province where you live. BC has the lowest marginal rates in Canada at the lower rates of income, over about $75,000 though you're better off in Alberta as we have a flat tax at the Provincial level.
Alberta is generally the best place to live from a tax standpoint in Canada as natural gas is subsidised, there is no healthcare premium from 2009 onwards and we have no PST.
Supposedly we have the best healthcare in Canada as well, which if it's true makes me wonder how people who live elsewhere in Canada survive.
Bear in mind if you're an MD then the healthcare system in Canada is nationalized, coverage varies at the Provincial level but there is a minimum standard all Provinces have to meet. If you fall into one of those occupations you have to work for the Provincial healthcare system, essentially. Dentistry, optometry, cosmetic surgery and so on are usually not covered except in rare situations (e.g. you're a burn victim).
There are also extra immigration requirements for MDs, assuming you want to practice in Canada, however there is a huge move on at the moment to certify foreign MDs ASAP due to the shortage of GPs. I wouldn't have thought coming from the US it would be terribly hard.
Steve.