Originally Posted by reverendfrag
What I cannot seem to find is a coherent description of what a higher temperature will do to the taste of my final product.
Unfortunately, that is because there isn't one good answer. It can do a number of things depending on your recipe, most importantly the yeast you use.
You really won't have an answer until you drink it. So don't worry, for now.
For example, a yeast like Wyeast 1056/ of WLP 001 will kick off esters, phenols, and higher alcohols at that level. This would be bad because you would usually use those yeast when you want a clean beer. But these compounds can be welcome in some cases, or hardly noticeable in others. It depends on what beer you are making as to whether or not it is "bad". Frankly, I would'nt ferment anything at 78 but a Saison, or specialty belgian.
As for diacetyl, the yeast will kick of diacetly if the temp is too high when you pitch the yeast. Funnily enough, the way to remove diacetyl is to raise the temperature at the end of fermentation, the yeast will reconsume the diacetyl. Likely, you won't have much of an issue here, but if you take a sample and it tastes 'buttery' then let it sit at a warmer temp for 2 or 3 days before you seondary or package it.