Being WordPress developer does not require special abilities, different from those of programming in PHP or CSS, but if there is tips that can make a difference and develop solid developments and, above all, without problems for him or his clients.
That is why I thought it convenient, because the tips are right, translate this list:
1. Always define WP_DEBUG to true during development
You can not imagine the number of code error notifications that you will receive from plugins and themes. All these low level warnings can cause memory failures, which will reduce the performance of the site by consuming more memory than necessary. WP_DEBUG is also a very useful tool to discover if you are using obsolete functionality.
2. Stay tuned to the WordPress code and CSS standards
The WordPress standards have been established after years of experience. If you abide by these rules you will get a clean and easily readable code for yourself and others. Hence the slogan "the code is poetry" ..
3. Comment your code
Your code should not only be easy to read by others but you should be able to let the rest understand why you chose to write a code in this or that mode. There are many ways to dress a saint, but explain your choice following the instructions in Codex .
4. Learn how the WordPress "core" works, read the actual lines of code
You should know how WordPress works internally. When are the plugins loaded in mulsitis? When are requests made to the database? When are the files loaded? and all that. You can learn a lot from reading the "core" of WordPress.
5. Use the functions and internal WordPress APIs whenever you can
Do not dwell on the wheel, before writing code learn if there is a function or procedure that is already in the "core". Learn from the "core", as I have pointed out before, and discover how everything works. You will be surprised by everything you will find there. Some examples would be these utility functions .
6. Put pommery to your code
There are many people who write code for WordPress in themes and plugins. And it's quite possible that your function name, or variable class, is the same as someone else's. For that reason you should always put pomp, like your initials or your website. For example (AW):
aw_display name ();
7. Never trust the user's action! Sanitiza & escape!
This is a major problem, now and maybe forever. WordPress is loaded with tools that allow you to write secure sites. A good idea would be the chapter 6 of this book . You already had it, right?
8. Use nonces
This would be an extension of the previous commandment. Using "nonces" ensures all the functionality your forms need. Review this Codex page
9. Locate your code
In Europe, unlike in America, we are very used to writing in several languages, so it is not surprising that a client asks you to have a theme or plugin available in another language or dialect.  So write your code thinking that it is "internationalizable". This will make adding another language in the future easier.
10. Performance, performance, performance