The time limits are imposed on at least two sides - provincial health insurance requirements, while they vary from province to province, limit the amount of time one can spend outside the province or be at risk of losing their coverage.
US INS limits any visitor (not just Canadians) to six months before one must leave the country, although there is a method for applying for extensions to this.
There are also limits placed on me personally because I collect OAS...CPP can be paid worldwide but OAS is a 'benefit' of living in Canada. I have to be present and accounted for in Canada for 183 days in each year.
The 'closer connection' rule is a somewhat complicated calculation of days spent in the US over rolling 3 year periods. For 2014 it's this:
31 days during 2014 AND 183 days during the period of 2014, 2013 and 2012, counting all the days of physical presence in 2014 but only 1/3 of the number of days of presence in 2013 and only 1/6 the number of days in 2012.
You can still qualify for the 'closer connection' exemption for US taxation if you maintain a tax and a physical presence in Canada.
For example, I can spend up to 182 days in the US in 2015 because I get paid in Canada, have Canadian bank accounts, pay Canadian taxes, have a Canadian place of residence etc. etc.
The only difference now is that I will have to file a form with the US IRS to prove that I am exempt from paying US income tax on my worldwide income (which is all Canadian). I'll have to do that now that entry and exit is being so closely tracked. The rules have always been in place but there was little way of enforcing them effectively.
I have the 2014 form on my desk and before I lose track of how much time I was in the US for 2013 and 14, I have written all the trips down for future reference.
Other than keeping track of how many days, the form is relatively simple. mostly checking boxes off and repeatedly writing Canada in multiple places...
Just more paperwork to be done tis all!