At the lowest level, the Snap Framework is not at all opinionated. The base level of the framework is comprised of snap-core and snap-server. snap-core is a web server API roughly on the level of Yesod's wai with maybe a little more slightly higher-level functionality. snap-server is Snap's web server that implements the snap-core API--roughly equivalent to Yesod's warp.

I personally don't understand the draw of scotty because as soon as you want to start doing more substantial web development, you're going to want higher level abstractions. The Snap Framework adds to this in a pretty flexible and not-too-opinionated way with snaplets which are implemented in the snap package. The snap package also contains a few core snaplets that almost every application will want out of the box like sessions, auth, and a snaplet for Heist templating. The Heist part is the only place where the snap package gets opinionated. But you are definitely not locked into using Heist. You could very easily use Yesod's Hamlet, HStringTemplate, hastache, or anything else. Heist just happens to be the direction that the core Snap developers seem to prefer.