Last year, Matt Brice wrote a nice article called Think Fast – Lessons for Dating, Investing. The article's message is that, whether in love or investing, people should eliminate bad prospects quickly so that “By spending less time with those who are not a match, you get to spend more time with the one who is a match.”
I can't comment on Matt's dating life, but this is great advice for investing. There are 10,000 publicly-traded companies around the world, far more than any one person can follow. Finding the ones that are undervalued, or are excellent businesses, means quickly filtering out the majority that aren't.
The article mentions a dozen kinds of investments he filters out out, including fashion retailers, scammy businesses like payday lenders and for-profit colleges, and companies that depend on a single powerful customer (e.g. Wal-Mart suppliers).
I agree with those, and I also like his broader point that each investor should make his own list. While some companies are intrinsically better than others, everyone has a different personality and different experiences, and investing success depends in part on finding investments that mesh well with those.
With that caveat, here's my list:
Fashion retail. I started life as a value investor and have gradually drifted toward macro investing. I don't think either methodology works for retail. Fashion retailers lack the margin of safety value investors seek: most don't have durable brands, and if they fail, their assets (leasehold improvements, ugly clothes) are worth little. And while recessions hurt retailers and economic growth helps them, they also can succeed or fail for reasons that have nothing to do with the broad economy.
Secular growth stories. These are hard to predict. Apple has experienced phenomenal growth and profitability since 2003, but from 1980-2003, its stock barely kept pace with inflation. Monster Beverage (formerly Hansen Natural) is one of the all-time great growth companies, but before its energy drink took off, it was a middling juice company that had unsuccessfully tried selling soy drinks and nutrition bars.
Declining businesses. I find these hard to predict too. A decade ago, I assumed GameStop would be out of business by now, but it's still minting money. When a business becomes obsolete and fails, there are always warning signs that it would happen, yet many companies have the same warning signs but manage to fend off obsolescence.
Venture capital. I'm a big fan of Jerry Neumann's writing. One arguments he's made is that some of the best venture capitalists have achieved success by embracing uncertainty rather than risk. E.g., they invest in companies for which the market opportunity is essentially unknown. That's the opposite of what I do: I look for situations that have definite historical precedents so that I feel the odds are knowable. VC seems to be full of opportunity, but doing it successfully would go against all the research I've done and habits I've developed so far.
Fraudulent companies. There's a well-known short seller named Marc Cohodes who publicly attacks companies he thinks are frauds. He has a strong record that includes exposing Lernout and Hauspie back in 2000. Unfortunately, many of his shorts double before they implode. He's also been sued by companies he's criticized. I think Cohodes is willing to endure these risks because he has the personality of a street brawler and enjoys fighting slimy companies. I don't and would hate to be sued.
There are other reasons why shorting frauds isn't attractive: it's popular among long-short hedge funds, so frauds usually have a high cost to borrow, and the rise of passive investing means that a growing number of investors don't care about business quality.
Health care. Health care is insanely expensive in the United States, and I think we're just one populist Democrat away from reforms that crush the industry's profitability. There's also a contradiction inherent to health care, and in particular pharmaceutical drugs: the most profitable products are rarely the most effective, while many of the best ideas get little attention because they aren't moneymakers. A cure has to be taken only once, so it's less profitable than a drug that alleviates symptoms but doesn't offer a cure and has to be taken for the rest of one's life.