An attempt to capture notable differences between implementations:
(I'm aware of Smith, but don't believe it's still active).
Include CFML and platform differences - syntax, feature, and extensibility.
Note, this is not something I've not seen anywhere else, but maybe I'm just not searching correctly. So links to that end would count too.
Update
Here's my definition of notable in this context: something that simplifies solving a difficult problem. For example, Railo offers the <cfvideo>
tag, which transcodes video - a non trivial task.
Something I would suggest is not notable: the Railo function ListAvg().
The closest you're going to get to a complete answer will be: http://cfml-engines.info/, which is a work in progress and doesn't have all of the information you're looking for. It's just too open-ended of a question.
Another place to look for the latest details on OpenBD: http://www.openbluedragon.org/manual
That documentation is pulled straight from the codebase so it's always up to date. (This is another way of saying the OpenBD wiki may not necessarily be the latest and greatest.)
You don't need to search A LOT. Both "alternative" engines have official wikis with articles specifically describing the compatibility questions:
Marking as a community wiki.
Overview
Aggregation of items I've called "notable":
Open Bluedragon
Railo
<cfvideo>
: video transcoding, pay feature, but low cost, free option is ffmpeg, but that has legal question markscluster
scope: interesting way of solving messaging between servers, pay feature.