Really spot on answer on catching "the usual suspects" of destined-to-fail business model. I tend to be skeptical when startup doesnt have clear monetization strategy from the start. Scale-first-monetize-later scheme only work with ground breaking ideas and whatsapp level of userbase. Since that condition is rarer than an unicorn, lets do more reasonable and understandable business model.
Answer by Lee Semel:
- Anything that makes programming "easy for non-programmers or businesspeople"
- T-Shirts (except Threadless (and Busted Tees … thanks Adam Kazwell))
- Recommendations based on what your friends like
- RSS readers, or more generally, sites focused primarily on presenting and formatting news stories gleaned from 3rd party sources
- Customized personalized newspapers focusing on mainstream news (as opposed to niche industry news with high business value)
- Find nearby people to meet/date through your mobile phone
- Anything involving paying people to look at ads
- Sites that purport to measure someone's trustworthiness as a standalone service, separate from any other context or functionality
- Craigslist killers – but not sites that attack individual categories on Craigslist (see diagram in this question's summary)
- To-do lists – there are tons of these, but everyone's workflow tends to be so personalized and specific to that person that none of them really catch on
- Most blogs that are started with the intent of being businesses
- Business that let consumers scan some kind of code, number or barcode in real life, with some special device, or lately their phone, and they get sent a URL, ad or coupon in return.
This isn't to say that these are terrible ideas perpetually destined to fail, rather that they are commonly attempted ideas, that been tried many times, and fail frequently.