What makes a good middle tier?

As computers evolved, so did the architecture of the systems that ran on them.
 We went from decentralized processing, with most computation done on the client,
 back toward centralization, with the more time-consuming and computationally demanding 
 operations moving back onto the server.

This shift goes hand in hand with a move to a multi-tier architecture with three 
 basic layers for presentation, application logic and data. Set aside the presentation and data layers,
 also called the front end and back end, and the heart of the architecture is the application layer, also called the middle tier. This is where 
 the business logic that controls the system’s individual processes runs.

Software solutions for the middle tier, known as middleware, include J2EE, PHP and .NET.
 Satya Komatinemi looks at what makes a good 
 middle-tier architecture in his article Qualities of a Good Middle-Tier Architecture.
 The software solutions above provide technologies for handling business, or application, logic. The quality of
 those technologies can have a direct bearing on the quality of the layer itself. Defining criteria for success
 and using them to compare the available solutions makes this a very good article.