Asset allocation is widely considered to carry the most weight in determining a portfolio's overall return, but we often avoid/ignore many categories that could be helpful. See article here for a discussion of why diversification matters, Fidelity.
On the contrary, though, some analysts believe too much diversification is not a good thing. See article here (WSJ) to help answer the question, "How Much Diversification Is Too Much?"
Dolvin, Steven D., "Diversification Revisited" (2014). All Chapters. 111.