In respect to diversification, fiduciaries are bound to show how they arrived at their decision on the following:
Asset class diversification - What was the process by which they decided on the asset classes to be held in the portfolio?

Diversification within an asset class - How many stocks will they hold and in what sectors relative to the index? 
What minimum diversification requirements will be mandated in the account guidelines for both stocks and bonds?
International diversification - What are the allocations and to which countries? What is the proportion of domestic and international holdings? What will the exposure be to foreign currency risk?

Alternative asset classes - What should be held in alternatives?  Are their liquidity issues if there's a need to reallocate large holdings in hedge funds and private equity?

Illiquidity - What is the budget for illiquid assets and how will this affect account rebalancing?

Concentrated positions - How are concentrated positions such as privately held business interests or employer stock and option holdings considered?

Return assumptions - Are return assumptions reasonable and based on future expectations as opposed to historical data?  Are illiquid asset return assumptions properly adjusted to reflect the fact that clients may not be able to access their investments in these funds for years?

Duty To Consider Taxes
Taxes are a critical consideration in the investment process and will be particularly important in the coming years.  Portfolio design should be based on the after-tax return of various asset classes.

Duty To Consider Fees
Fees matter, despite the fact that clients often invest in portfolios without carefully examining the fees they will be charged.  Since fees reduce returns, they should be part of the portfolio design process.  Advisors should explicitly explain to clients how fees are taken into account in designing their investment plans.  Also, advisors need to disclose their compensation for each of the portfolio alternatives they are proposing.

Duty To Be Loyal, Impartial And Objective
A fiduciary advisor is required to act in only the client's interests when making investment decisions, and to provide an objective opinion. The advisor will be required to show how his actions are designed only to benefit the person or persons for whom he is investing.  To ensure this obligation is met, fiduciary investment firms generally have committees that control and document the investment decision-making process.

Along with this responsibility is the duty to avoid self-dealing-when a trustee or other fiduicary takes advantage of his position and acts in his own interests rather than those of his clients.