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.

The same code can be used for web, server, desktop and mobile.

Imperative, OOP, functional programming in one language.

Nim has a lot of common with Python in terms of syntax. Indentation-based syntax, for/while loops.

There are generics, templates, macros in Nim. They can allow you to write new DSL for your application, or avoid all boilerplate stuff.

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.
Also, you can use Nim with Objective C or even JavaScript (if you're compiling for these backends).

Checks your code at compile time.

You can use unicode names for variables, there is "unicode" module for operations with unicode.

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).

With built-in "unittest" module you can create test with a very readable code.

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.

Nim has "asyncdispatch" module, which allows you to write async applications.

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()