Green card holder here.
The actual answer spans nearly all but the last entry; for some people it’s a slamdunk, for others it can be a nightmare, for most it’s in between. The paths to citizenship can vary wildly.
I took the employment sponsorship path, which took about three years to GC and will end up taking eight years (total) to citizenship. A friend of mine here in LA married an American, and will beat me to citizenship by two years. If you have a U.S. parent, you can go straight to citizenship in relatively little time; if you are sponsored by a U.S. relative other than a parent, it can take a very long time, >5 years.
The two big things that I can point out that matter the most are these: DO have a degree OR analogous work experience (this would include being published, and having your work lauded by peers), and DON’T have a criminal record.
Past that, there’s a lot of odd loopholes and difficulties; if you are a leftist intellectual who pillories America left and right *but is applauded by other intellectuals*, it counts in your favor; if you lack a degree and have not yet made a splash in your industry, getting in can be quite the legal hack (in my case, I had to spend $12kCDn on some classes in the States to upgrade my degree, and that was *with* a job offer from a well-respected company. And before 9/11).
Once I had the visa and had proven myself to my employer, the Green Card process was long in the paperwork and legal fees (not borne by me, thankfully), but the actual entry interview was a trip to Montreal (consular processing, usually the best bet), three hours of waiting and then two simple questions in a small room, and done! (the agent actually said that this was going to be his shortest interview on record). Then some final extra processing at the entry point to lock in the new status, and fini.
Regarding visas, if you are a Canadian or Mexican, there are certain special visas issued under NAFTA that do not have quotas. These are a good initial step, but a good attorney can guide you around the pitfalls.
Absolutely, positively, have an attorney. One bad step can get you barred for a year or more.
As for the bureaucrats, there is always the risk of tangling with one who sees terrorists under every rock or simply feels like throwing some weight around that day and he doesn’t like your nose. But most of them simply do not like being knocked out of their routine by having to process exceptions. They don’t like that. That means, don’t forget to dot your i’s and cross your t’s, have every bit of documentation that is required, and be sure of every fact you give them. Help them pigeonhole you; that way their ass is covered if anything goes awry, and that’s all they want as a minimum. I found that out the hard way going into Detroit, and that was just a business trip.