Besides the reference doc (which is a good way to learn a small language IMHO), there's Programming in Lua (PiL). It teaches both the language and the common idiomatic patterns.
About scope, Lua is a high level language. It allows you to quickly express algorithms and create structures. It's also widely used as an extension language, to be embedded within an application.
There are many C modules available, both to add new capabilities (LPEG, LuaSocket, etc.) and to 'bind' to existing C libraries (databases, XML parsers, etc.), but Lua itself is a complete language, you're not required to write C modules on day-to-day work.
About the "most common design patterns" do you mean the "Gang of Four" design patterns? (you mention the Observer pattern). If so, While really useful and written in a language-agnostic tone, many of these patterns make sense only in statically-typed, heavily OO languages like C++, Java, C# and a few others. With other platforms, some are unnecessary (like the Command Patterns, which becomes a closure when you have lexically scoped, first class functions), some others are applicable but result in widely different solutions.
About multithreaded programming: Lua itself is mostly single-threaded. There are several ways around this, but they're not so heavily needed as on other platforms. A simple coroutine-based scheme is quick and easy to write and works perfectly in many cases. When not, the exact way to do multithreading depends a lot on your needs.
GUI: this is in the realm of a GUI library and not of the language. There are several available. Again, you can pick any one, or none at all. There are many things to do without GUIs (command line, application extensions, servers, etc)
Hardware integration: depends on the hardware. In most cases, there should be a C API and so you need a binding library, which is not hard to write (sometimes it's just a 5-20 line C function for each API entry point)