Fermentation of a typical 5 gallon batch of beer can generate as much as 50 BTU/hr during the early stages. It is not uncommon to see 5 to 10 degF increases in temperature in very well insulated fermentors. I have found that it contributes to "homebrewish" flavors. The other replies in this post explain the potential off-flavors very well: esters, fruity, hot alcohol, solvent-like, fusel alcohols, etc. I tightly control my fermentation temperature by submerging my glass carboy in a temperature controlled water bath. My beer has a very clean flavor profile, I can't emphasis fermentation temp control enough if you are trying to make a high quality, professional tasting brew.