The editor GUI is very powerful and intuitive. It allows pausing gameplay and manipulating the scene at any time as well as progress gameplay frame by frame. It also has powerful asset management and attribute inspection.

This allows it to be more powerful than other, simpler drag-and-drop engines such as Game Maker Studio, although it can take a bit more experience to learn the workflow.

Can be used for projects of any size. Anything from basic mobile games to big blockbuster triple-A titles.

Games have been made in Unity at all levels of the Video Game industry. If you want to make games, this is a great engine to learn. Learning Unity will teach you the basics for any engine, and if you want to get a job at a big studio, there is a chance that you will be working on this engine there as well.

Great Community support through Forums and Unity Answers.

It is one of the biggest and best tools available, and there are a lot of tutorials available to help learn how to make great games. Even without knowing how to program, there are assets in the Asset Store that can help you by allowing linking scripts inside the game engine.

3rd party IDEs, like Visual Studio, can be used to write code for Unity.

As long as the development company makes $100k or less, it can use the free version of Unity to release games.

The way the editor is structured, by setting scripts on objects, and the use of a high-level language, C#, makes it easy to learn.

Assets can be imported from many different formats including .FBX, .OBJ, .Blend, and .Max.

Unity is a proven game engine. It is used by a wide range of developers - from small indies to triple-A companies such as Microsoft, Paradox, Square Enix and Sega.

Unity offers over 20 platforms for publishing including mobile, console, web, VR, and more.

Unity provides a great state machine animation system called Mechanim allowing to separate animation from the model and assign the same animoations to different models.

The built in standard shader in Unity 5 is incredibly optimized and supports PBS/PBR.

Unity runs very smoothly even on systems that are considered "weak" by today's standards.

Unity provides a selection of programming languages depending on preference or knowledge. C# (CSharp) is arguably the most powerful, while JavaScript has the widest selection of tutorials. It should be noted that it's probably best to avoid Boo (a flavor of Python).

As long as the company makes $100k or less, Unity's free version can be used to release games without purchasing the pro version.

Once a license is bought, there are no additional costs that need to be taken into consideration.

For those developers who can't afford an artist, or aren't skilled enough to create their own art, Unity features an Asset Store full of a wide variety of free and paid assets that can be easily added to a game. The Asset Store has more than just music and art. It also has code and modules that can be added to games including unique lighting or GUI systems. It also has powerful asset management and attribute inspection.

Unity allows free mobile development. There are some limitations.

Unity's modular system and usability allows for quickly developing a prototype of an idea. It has features like drag & drop editing, shaders, animation and other systems already in place to allow diving right into developing a game.

Unity3D provides an exhaustive documentation where everything is given a full description supplied by a number of examples as well as video and text tutorials and live training sessions to understand the ins and outs of the engine. In addition there's an ever-growing community that can offer advice to help resolve any situations that may arise.

Along with the official Unity resources, there are many high quality (and often free) third party tutorials available.

Overall, a coherent engine with a rational approach. People who complain a lot about being forced to hack around it usually do not read the docs, like the one that describe orders of execution, or specific functions hooks and such. Some like to say it lacks raw power where people who are used to standard optimizations have no problem. For example It is not uncommon to encounter users who complain about low FPS but forgot to activate occlusion, flag static elements, activate animations culling, and so on. As for complaints about C#, people who are transitioning from C++ were already bad at C++ before being bad at C#. They often come from the PC world where the sheer power of today's machines is very forgiving compared to the platforms we had to develop for in the 80s~90s. One of their errors is for example to never read this doc.