When comparing Nim vs Qt / C++, the Slant community recommends Qt / C++ for most people. In the question“What are the best languages to write a desktop Linux application in?” Qt / C++ is ranked 3rd while Nim is ranked 14th. The most important reason people chose Qt / C++ is:
Professional, thorough documentation with examples, available either in a web browser or in the stand-alone desktop client called assistant.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
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 Has built-in unittest module
With built-in "unittest" module you can create test with a very readable code.
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 Has built-in async support
Nim has "asyncdispatch" module, which allows you to write async applications.
Pro Really cross-platform
The same code can be used for web, server, desktop and mobile.
Pro Easy to read
Nim has a lot of common with Python in terms of syntax. Indentation-based syntax, for/while loops.
Pro Multi paradigm
Imperative, OOP, functional programming in one language.
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.
Also, you can use Nim with Objective C or even JavaScript (if you're compiling for these backends).
Pro Garbage-collected
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 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 Built-in Unicode support
You can use unicode names for variables, there is "unicode" module for operations with unicode.
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 Good documentation
Professional, thorough documentation with examples, available either in a web browser or in the stand-alone desktop client called assistant.

Pro Portable
Linux, BSD, Android, iOS, Windows, Mac, Blackberry ... Qt has 'em all.

Pro Global community
The community behind Qt is both massive and approachable. Digia (also owners) are joined by the likes of Intel, KDAB, ICS, Canonical and numerous others in sponsoring development, while communities such as KDE also contribute significantly. Forums are active, mailing lists are open, irc channels chatting, git repositories well managed. Answers to questions are usually minutes away.

Pro QML
QML is "HTML5 done right": a declarative language that integrates beautifully with C++ code when necessary and exposes the power of the GPU ... simply the best way to create a modern GUI in terms of effort and results.
Pro LGPL license
Pro Open Source
Qt is licensed under a OSS license through a dual-license policy.
Cons
Con Not free/expensive
Free version is limited and prohibitive.
