I don't know if it's flammable or not or just supposed to be up to DM discretion. I don't know if it always produces a substance with the same properties, or could it be different from one cast to another. I also don't know if magical Grease is the same as non-magical grease.
Jeremy Crawford has stated that if it were flammable it would say so. I can't figure out if this means that it's not flammable, or that it's not necessarily flammable.
All we know is that it's slick and that it's called grease. We aren't specifically told if it's opaque, translucent, or transparent, but we can't assume it's not any of them just because it's not explicitly stated.
I feel like we are supposed to assume it's not necessarily flammable, and that it's up to GM discretion, but I have no idea if that's the case, or if I'm missing something important.
So beyond the name of the substance, and knowing that the substance is slick, what, if any, other properties does grease (specifically the magical grease produced by the spell Grease) have?