AGNC Investment (AGNC 2.37%) is not your typical real estate investment trust (REIT). It doesn't own physical properties, but rather invests in a portfolio of mortgage securities. That makes it more similar to a mutual fund than to a traditional REIT, and the difference has material implications for investors. If AGNC Investment's massive dividend, which currently yields 14.2%, looks tempting, there's one big factor you need to be aware of.What's a REIT worth?Technically, there isn't a big difference in the ...