There's so much that could be written about the MOD creation process. The article covers some concepts but there certainly could be more. For example, before undertaking a MOD I like to try to anticipate how complex the MOD will be. Is the MOD going to extend / alter an existing feature or add something completely new? To me it's far easier to write a MOD that alters how something works because the basic functionality is already there.
Another measure of complexity is to figure out how many core phpBB files will be affected by the MOD. Some MODs that seem simple might turn out to be more complex simply because they impact so many areas of the code. In the old phpBB2 days adding a new profile field was one of these. That issue was addressed by allowing custom profile fields to be defined via the board admin in phpBB3, which is a far better solution.
MODs that require database alterations are obviously more complex than those that reuse the existing structure. MODs that alter existing tables (adding columns, usually) are likely to be easier than those that require entirely new database structures, primarily because the MOD author doesn't have to think about index structures and database tuning since the MOD will inherit the existing relationships.
Testing ... that could be an entire blog post. The first rule of MOD writing should be not to break existing functionality... unless that was the intent of the MOD.
Tuning... that's another interesting concept that could be an entire blog post. Something that works okay on a test board might fail miserably on a board with several hundred active users or hundreds of thousands of posts. I remember when phpBB3 was entering the final phases of development the used the database from phpbb.com as well as some other large boards as test cases to look for performance issues. Performance tuning can revolve around database performance tuning (indexes or table designs), efficient php coding, or feature redesign if needed.
Security... yet another concept that could be discussed around MOD writing. Things are easier today than on the phpBB2 framework (if it's not obvious, I'm from the old-school days and am still more familiar with phpBB2 than phpBB3) because security was a major concern for the core of the phpBB3 code. There are new functions that address SQL injection and form input values, and every MOD should use them rather than reinvent the wheel. In addition to these coding techniques, however, the MOD should be sure not to break / invalidate the phpBB3 security configuration. It should respect group and user permissions to avoid unintended consequences.
So much for the rambling.
One final note, however, is that I would personally put the creation of the MOD topic far sooner in the topic than as was written in the blog post. This is purely a matter of my own opinion, but I like to start soliciting feedback and input from potential MOD users as early as possible in the process, mainly because it's easier to change the MOD design before any code has been written.