While investors can use ETFs to go triple long or triple short or invest in specific countries or commodities, some products should be avoided.ETFs empower investors by allowing them behind the velvet rope of investing and into a world of exotic strategies. While many of the narrow and broad ETF strategies can help investors transform their investments, the answer is never "one size fits all." Every day I work with investors to assess which ETFs fit their needs to design custom portfolios that work. In the course of these discussions, I often find myself returning to basic tenets of ETF investing. I have detailed three of the most important: appropriateness, liquidity and concentration.