The biggest U.S. stocks haven’t been this expensive since the dot-com era. That’s making investors nervous. - MarketWatch

1. U.S. large-cap stocks near highest valuations since dot-com era. 2. 89% of fund managers view U.S. stocks as overvalued, highest since 2001. 3. Retail investor sentiment shows 47% expect stocks to decline in six months. 4. S&P 500's forward price-to-sales ratio equals levels last seen in 2000. 5. High valuations raise concerns of vulnerability to negative news.