Many of the features I think will be a great addition to the website and some, not so much, but alas what is good for someone is not always good for the next guy.
Out of the features listed, I think that being able to search and filter results by transmission type and engine size or trim level will be a biggie for me as I drive a manual in a car that is usually dominated by automatics (e90 325i) and i'd like to be able to compare my results to others with the manual transmission only. Or with my last car, I had a Grand Prix SE with a 3.1L engine which was rare to find. Most others had a 3.8L and it was a lengthy process to find the others with a 3.1L engine to compare numbers with.
Adding the ability to add in the octane, fuel additives, and maintenance I believe is a good thing as well. As would if we could specify which gas station we were at to compare our MPG from one gas station to the next, especially today when one gas station might add 10% ethanol and the one next door might only add 5%. I use aCar and Gas Cubby myself in addition to Fuelly and while it might be hard to compare the maintenance between users, it is fun and helpful to see it all digitally on one screen to see how much you spent, the life you got out of certain wear parts, and the such without sorting through messes and stacks of papers. And if the option to record a fuel or oil additive is added, I think that the ability to search site wide for the people who do so might be interesting. That would allow you to see if any additive actually works and to what extent.
Some of the things that I think are a bit much is the temperature and elevation, although once enough people have recorded fill ups with these features, the ability to see the difference between sea level and a mile up or in a hot versus a cold climate could be interesting.