What is a commonly used rule of thumb for residential apartment buildings of 8 units and larger?

You will want to look at many things. To start find out what the "going" cap rate is in the area. Analyze the IRR (internal rate of return) of a property 5-10 years into the future with a conservative future sale value (I generally use the same cap out as the purchase cap). Keep in mind whatever you use will be wrong because it is the future and many things will cause unanticipated change. What the IRR analyzation will do is compare one property to another. Be sure and use the same parameters so you are comparing apples for apples. IRR allows all kinds of variables (such as resnt increases) over the entire period of investment while cap rate only looks at a one year window without any debt. IRR on the other hand will take into account variable cash flows and debt.

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I say having good management in place. Multi-families are immovable so the manager has to be able to retain as well as promote to get new tenants...replenish the community to reduce vacancies and increase gross income. In addition, having and maintaining amenities fit for enjoyment is a plus.