Remarks about the three implementations completed thus far:

Tornado

• Least disliked version (as a whole) thus far
• Least painful to develop (no surprise; I’ve worked with Tornado previously)

Flask

• I’d gotten rather used to class-based views; they were missed here
• I enjoyed using the Jinja2 template engine (certainly more than the Tornado template engine)
• Lack of included batteries seems like a major step backwards after using Django and Tornado

Flask with SQLAlchemy

• Nope
• Flask-SQLAlchemy package made using SQLAlchemy less painful, but, as a whole, Alchemy is the least pleasant ORM I’ve used

Up next: Spring (Java). I look forward to using it properly with Maven or Gradle, a nice change of pace from work where those tools are curiously verboten. [Edit 2/20/15: Maven is no longer disallowed by my employer.]