It's definitely easier (and more profitable) to pick a small niche and become super good at it... with some effort, you can become a "category killer" in some very narrow topic. One of the benefits (I have found) is that it's not worthwhile for large merchants to become deeply specialized... so individual sellers can rule niches; we should let WalMart be WalMart ("general" merchants) because they are already far better at that than we can ever hope to be! Good post!