1) All good software engineering is fundamentally motivated by KISS and YAGNI.
It's a fairly straightforward observation that the complexity of software increases as functionality and flexibility are increased. This manifests itself across the codebase. You can see it in the basic interfaces and APIs, in the user interface, and in the checkbooks as you spend millions of dollars maintaining a mess of super flexible goo of code. Good software strives for simplicity, and elegance, not infinite configurability. Don't try to design systems that solve every conceivable problem that could arise. If an unforeseen problem does arise, refactor to it, keeping things as simple as is reasonable.2) A wide range of experience in multiple languages, paradigms, and environments is critical to a developer's ability to design software.
Every developer should know a large variety of languages. Lisp, Ocaml and/or Haskell, assembly, and C are on my list of important languages to know. Personally, I would not advocate developing in any of these languages (well, maybe Haskell), and I give my condolences to those of you who are forced to. Still, it's important to know these languages and many others because they give you a wider pool of knowledge and ideas to draw on. Many modern ideas in C++ are borrowed from Lisp, albeit in a rather contrived and roundabout manner. Software engineering isn't just about knowing the language you happen to be using and keeping a copy of Design Patterns on your desk. The more languages you learn, the more tools you have at hand, no matter what you're developing.
3) There are no legitimate uses of the singleton design pattern.
And I mean it. There is no correct way to use a singleton. There is no situation in which it makes sense. Not for logging, not for resource managers, and certainly not for your renderer or related objects. Everyone I've talked to has been a little bit hazy here, saying that maybe, in a particular scenario, a singleton could be a good solution. I am rejecting that outright. Singletons are a useless antipattern. If you have one, your design is broken.If you want some actual discussion of these points, feel free to post a forum thread. I just wanted to list the points here; each one merits a discussion of its own.
[EDIT] Reordered the principles.