It's defined by an ANSI standard; see (
http://clhs.lisp.se).
There are a handful high-quality implementations, both free and commercial, many of which can generate efficient native code. When you can stick to the standard, you can reasonably expect your code to work across implementations and platforms.
The existence of the standard also means that you can often use code that is decades old with few changes. Furthermore, there are actually bodies of old code that you might want to use, because of its relatively common use in AI and academic communities in the 80s and 90s. See, for example (https://www.cs.cmu.edu/Groups/AI/0.html).
It's designed to support interactive programming to a greater extent that any other language except perhaps Smalltalk. For example, the ANSI standard defines functions you can use to change the definitions of classes at runtime and have existing instances automatically updated to use the new definitions.
Like Racket and Chicken, it has a good library ecosystem. The ecosystem is supported by a good package manager (https://www.quicklisp.org/beta/). That package manager is in turn supported by a decent documentation-search site (http://quickdocs.org).
Several Common Lisp implementations support easy delivery of programs as a single, self-contained executable. The buildapp library (http://quickdocs.org/buildapp/) shows how this may be done in a cross-implementation way by supporting both SBCL and CCL.
The Common Lisp community has been around for decades, as have some of its prominent members. There's a ton of useful, practical knowledge in the community. It does have a bit of an unfriendly reputation, and that's sometimes deserved, but with some patience and persistence you'll find that it also contains friendly and helpful people who know a lot about the language and its effective use.
You can find a guide to the community here (https://common-lisp.net/community).