Ranked in these QuestionsQuestion Ranking
Pro Great metaprogramming features
There are generics, templates, macros in Nim. They can allow you to write new DSL for your application, or avoid all boilerplate stuff.
Pro Strict typing
Checks your code at compile time.
Pro Compile-time execution
Nim has a built-in VM, which executes macros and some other code at compile time. For example, you can check if you're on Windows, and Nim will generate code only for it.
Pro Really cross-platform
The same code can be used for web, server, desktop and mobile.
Pro Built-in Unicode support
You can use unicode names for variables, there is "unicode" module for operations with unicode.
Pro Type interferencing
You only need to specify types in your procedures and objects - you don't need to specify type when you're creating a new variable (unless you're creating it without initialization).
Pro Multi paradigm
Imperative, OOP, functional programming in one language.
You don't need to deal with all those manual memory allocations, Nim can take care of it. But also you can use another GC, or tweak it for your real-time application or a game.
Pro Has built-in unittest module
With built-in "unittest" module you can create test with a very readable code.
Pro Easy to read
Nim has a lot of common with Python in terms of syntax. Indentation-based syntax, for/while loops.
Pro Easy to integrate with another languages
You can use Nim with any language that can be interfaced with C. There's a tool which helps you to create new C and C++ bindings for Nim - c2nim.
Pro Supports UFCS (Unified Function Call Syntax)
writeLine(stdout, "hello") can be written as stdout.writeLine("hello")
proc add(a: int): int = a + 5 can be used like 6.add.echo or 6.add().echo()
Pro Has built-in async support
Nim has "asyncdispatch" module, which allows you to write async applications.
Con Still in pre 1.0
Not very stable and has a rather small community.