Well according to the oxford dictionary a coaster is "A fairground attraction that consists of a light railway track which has many tight turns and steep slopes on which people ride in small, fast open carriages:"
So by that definition a coaster must have a) Track similar to that of a railway b) Stuff that happens, drops/turns/slopes and c) Open carriages. So, a monorail isn't a coaster because the carriages aren't open and the track isn't like a railway. Powered coasters fit under this definition because gravity is not mentioned, however a water coaster should not count because it does not have track all the way around, this includes nautic jets. So on the topic of butterfly coasters, no they do not have track similar to a railway, yes they have a drop and yes, they are open. So I think that technically they are not coasters, but using the definition/catogorisation I have created, disk'o's are technically coasters. So really, you need to decide whether you are going to use my definition of what a coaster is, or the traditional "does it coast", because using the tradition definition will cause many clearly non-coasters to be coasters, while using my definition is clearly more restricting, for example if you remember back when legoland Windsor added those stupid lids to their mack thing, that would make it not longer a coaster as it is not open.