TMTOWTDI is a Perl-ism, standing for "there's more than one way to do it". It is often a blessing, particularly in the context of Perl, in that it results in multiple synonymous ways to accomplish the same task, each of which fits best in a different context.
However, as you might expect, this is a double-edged sword that can lead to messy code and a difficult learning curve. It's this negative context that bites the XPages platform in a number of ways.
One way is relatively benign but still a pernicious issue: the Java base of XPages contains a plethora of may-or-may-not-be-important choices to make, such as Vector vs. ArrayList vs. LinkedList. Though it doesn't really matter in small cases which you pick, you still have to make that choice, and some of them (Vector) end up hurting you as the scale increases. That's not something you have to pay too much attention to in most other high-level languages, where there's a standard Array type that does its job well.
The larger issue is the massive divergence of ways that even normal XPages apps can be structured. Do you use one main XPage and then put everything either in other XPages brought in with "include page" or in custom controls (with or without Dynamic Content), or do you do a lot of top-level XPages the user visits directly? If the latter, do you still break out chunks of individual-page functionality to custom controls for readability? How about data - do you store your data in the same NSF as the code, in separate NSFs, or in a non-Domino database? Do you use stock controls and make a OneUI app, toss all that OneUI/Dojo stuff aside and build on Bootstrap/Foundation/etc. and jQuery, or take a hybrid approach? For back-end code, do you use inline SSJS, primarily SSJS libraries, or Java? If you base it on Java, how? Controller classes, object data sources, phase listeners, variable resolvers, core business-logic classes called by SSJS? If you put your data access in Java classes, do you build them to work alongside stock XSP Domino data sources for performance, or build on the Domino API?
The list goes on.
There are likely as many divergent approaches to XPages development as there are XPages developers. While this is good in the sense of feeling out the contours of the platform's capabilities, it makes it difficult to get started and to collaborate with other projects.
As you might expect, I have my own opinions on each question, but some of them are still fluid. This is a natural by-product of a still-relatively-young platform with a strange history, no clear single voice, and a primary target market of ancient systems with massive inertia. In time, I hope that this settles down to a good general-purpose set of techniques (and I hope to influence that). Each application I make feels more coherent than the last, and the differences between them are gradually diminishing. The more consistent the methods (assuming they're good), the better for the platform and the developers on it.