Most people invest in order to secure their retirements.
Many countries, including Canada, encourage investment in Tax Deferred investment vehicles. (RRSP, TFSA, RDSP, etc.). U.S. tax laws punish the deferral of investment income. With the exception of the RRSP, most investment vehicles available to your neighbors will not be available to U.S. persons.
In addition, under no circumstances should you invest in non-U.S. mutual funds. (The only possible exception is if the mutual funds are in an RRSP where the proper 8891 election has been made).
Non-U.S. mutual funds are PFICs and should be avoided at all cost. If you own Canadian mutual funds (or ETFs) you should consider selling them. And if so, you should sell then in 2012 before the Bush tax cuts expire.
Bottom Line: As a U.S. citizen abroad, you will be subject to extreme limitations in how you can plan your retirment.