Ranked in these QuestionsQuestion Ranking
Pro Lets you focus on the content
LaTeX handles the design so you can focus on the content
Pro High-quality typesetting by default
There's a reason that scholarly journals often require the use of LaTeX for articles printed in their pages, and it's because the quality of the output is that good. Universities often require, or at least encourage, the use of LaTeX for graduate theses and dissertations for this same reason.
Pro Free open source software
Licensed under the LaTeX Project Public License
You can edit LaTeX sources in any text editor.
Works on every major OS and gives exactly the same quality output everywhere you go. LaTeX on macOS, Windows, Linux, BSD, and even Mac OS 9 has exactly the same output for a given set of sources.
Pro Effortless math input
The whole reason that TeX -- and, by extension, LaTeX -- exists is to give people an easy way (well, for some value of "easy") to produce high-quality documents with properly laid out mathematical expressions and text in them. As long as you know the language (or have a reference sheet handy), you can include mathematical expressions in your document with little to no extra effort needed on your part.
Con Steep learning curve
LaTeX is not what you'd consider easy to use, and while there's plenty of documentation out there, much of it is rather opaque unless you're a seasoned TeXnician.
Con Single-threaded design
LaTeX is single-threaded by design, since it must necessarily work sequentially to produce each page as it is laid out by the typesetting engine. This makes it dependent on the power of just one individual core in your multi-core computer setup and so migrating to a machine with more cores won't necessarily make your LaTeX documents build faster.
Con Not a what-you-see-is-what-you-get editor
LaTeX uses the paradigm what-you-see-is-what-you-mean instead.