ROTH IRA Investments:
1) Corporate Bonds: Interest gained on corporate bonds is fully taxable according to your tax bracket. Same with interest on foreign bonds and also CDs. Note that these types of bond mutual funds pay non-qualified dividends.
2) REIT Preferred Stocks: These dividend payments are non-qualified just like REIT common stock dividends. The difference is that preferred dividends are typically 100% non-qualified without the return of capital component.
3) Canadian Corporations: The last time I checked, Canada withholds a 15% dividend tax on US investors. An easy way to get around this pesky withholding tax is to place Canadian corporations in a ROTH. It is my understanding that this applies to corporations only (not Canadian royalty trusts), however I do not have any experience or references to back up that claim.
4) REITs: Real Estate Investment Trusts pay non-qualified dividends. Since this income will be taxed at your full marginal rate, IRAs make sense here. However, many people fail to realize that part of the typical REIT dividend is classified as a return of capital. You will not be taxed on the full dividend in most cases. Here is an example:
Realty Income's dividend was 25% ROC for tax purposes in 2012. The other 75% was fully taxable at the marginal rate. With that in mind, REITs are well suited for a ROTH, but not the highest priority.
Taxable Account Investments:
1) Municipal Bonds: Muni interest is always exempt from federal tax and sometimes state/local tax too. No reason what so ever to put these types of investments in a tax advantaged account!
2) Master Limited Partnerships: To be honest, the tax implications here are fairly complex. "Experts" do not always agree. In my experience, MLPs pay a return of capital which is not taxable but instead reduces the cost basis of the position. There are some other things going on, but effectively income taxes can be deferred until the asset is sold. Since MLPs are inherently tax advantaged, it makes sense to hold them in taxable account in order to delay the tax bill as long as possible (perhaps decades). Also under certain circumstances, it is possible to owe a tax bill with a MLP held in an IRA. Best to avoid that scenario.
3) Most Foreign Stocks: Most foreign countries slap a withholding tax on dividends paid to US investors. However, it is possible to claim a credit on this withholding during income tax season. In order to claim the credit, it would be wise to place foreign stocks in a taxable account (I don't believe it can be claimed in an IRA). Some countries, the UK for example, do not withhold dividends at all. As described above, Canadian Corporations can be held in a IRA/ROTH to avoid the annoyance all together.
4) Government/Agency Bonds*: Investors can side step state and local taxes with government bonds interest which is taxed at the federal level only. This tax saving feature is redundant in retirement accounts*! Be sure to do your homework with government agency bonds as some have tax advantages, some do not.
*Certain states do not levy income taxes. Tax saving benefits of government bonds isn't applicable for residents of those particular states.
5) Dividend Stocks & Non-REIT Preferred Stocks: Take advantage of qualified dividends by placing them in a taxable account.
6) Speculative/High Risk Stocks: Personally, I would put speculative ideas such as FB, AAPL, TSLA, LINE/LNCO, and most tech stocks in a taxable account in case of a capital loss. Ideas do not always pan out, at least realizing capital losses in a taxable account would soften my tax burden. Realizing capital losses in an IRA is not beneficial in any way!