1. Lethargy during a sideways market
2. Disregarding your strategy
3. Chasing somebody elses dream
4. Believing somebody elses FUD
5. Using money that you shouldnt be
6. Failing to properly do a self risk analysis before investing
If you can cope with these dangers then this type of investment will do well for you. This is a hands-off, long term method of investing. DYOR, find good coins with bright futures, buy and hold. Dont worry about the daily percentages
I don't believe in some of the point you mentioned above as that to me is a miss leading of people into making looses in their trading.
I don't think there is any denger in holding Bitcoin or some other top altcoins since most of them react positively with Bitcoin market increase.
Better than that mate, we must expect more awesome results as we prefer holding a promising asset. It's really difficult to find the best of them, but even if we can't choose the best at least we got one of them. The only danger of being a hodler is that you bought at high price, but eventually went down of its price. We probably tend to panic after price fall down, due to fears that it won't recover it's value.