The Web2py framework is an abstraction paradise. Databases hide behind a Database Abstraction Layer (DAL). Web2py's rendering system will try to find a view that -- depending on context -- displays an object in HTML, XML, JSON, or any of the half-dozen protocols the framework supports. Intelligently crafted by a professor of computer science, Web2py's template system actually lets you use Python as an HTML-embeddable scripting language. Falling somewhere between the all-encompassing world of Zope 2 and the minimalism of Pyramid, Web2py may well be the best framework for Python-savvy developers to enter the world of framework-based Web application development.
The philosophy of Web.py -- a minimalist framework -- is not to abstract away the details of interacting with the Web, but to make that interaction easier. As a result, you'll find yourself writing HTTP GET function handlers directly. Likewise, the Web.py database system does not abstract away SQL; rather than hide the fact that you're querying a database, it hides the details of working with different databases. Web.py does define a template language, which -- like that of Web2py -- lets you embed arbitrary Python code in a Web page. Web.py is ideal if you're already familiar with building Web applications (perhaps you once wrote CGI-based applications). You'll get started quickly with Web.py, but you'll have to rely on your own wits to go beyond simple Web applications.
Pyramid is also minimalist Web framework, not so much in its capability as its philosophy. It makes no assertion concerning the back-end database you should use, nor does it foist a particular template system on the developer. (Currently, Pyramid supports two, though Pyramid itself tries to remain agnostic regarding the choice.) If Pyramid has any blemishes, it is the quirky terminology with which its documentation describes the framework. In addition, its laissez-faire attitude can leave you wondering precisely how to proceed to accomplish a particular task -- and, when you manage to accomplish that task, whether your solution was the best. For example, there are two separate means of determining how your application will handle a given URL: the well-known URL-mapping mechanism and something called "traversal" (which you will need to read the documentation to figure out). So, with Pyramid, you purchase flexibility at the cost of turning yourself into something of an explorer.