I first decided that I wanted to make games when I was at a very young age. I started out with HyperCard (why I even tried with this is beyond me), and moved up to some shareware programs that generally didn't produce anything spectacular. The day before I found out about Unity3D, you could have shown me my latest games and I would not have belived that I made them; but now there is no limit to what I can imagine with this Macintosh Game Engine. So, lets jump right in with the features!
Unity3d offers a host of great features, even in the Indie version. Right out of the box you get a pile of useful assets, plus a Terrain Engine, Physics Engine, Networking, and support for nearly every major file format imaginable. Unity also supports 3D sound, lets you write your own Shaders (to change the appearence of ingame objects), has multiple kinds of lighting (with flares, halos, cookies, etc), and more. Unity definitely doesn't lack in the features department.
While the Unity Editor is only available for Mac OS X, you can deploy your games in multiple ways. First, there's The Standalone Binary for Mac (which can be PPC, Intel, or Universal; at your call). Then there's the Web Player (which can be streaming or not), and allows anybody with a Mac or a PC to play your game on the internet as if it were a Flash video (all they have to do is install the Web Player Plugin, which is free and fast). Then you can deploy for Mac OS X's Dashboard as a widget (though consumers here will still need the web player plugin). If you pay for Unity Pro, which I'll talk about later, you can also deploy as a Windows standalone. There's no Linux support for Web Player or standalone, but that's life, right?
The Unity Community is Unity's official forum. I'd have to say that the general additude and knowledge level or members there is through the roof positive, and they will be happy to help you with any problem you have. In addition to the incredibly helpful community, there's the Unify Community Wiki; where people can post assets (scripts, shaders, special effects, plugins) that they've made for other community members to use. Unity Technologies (the company who makes Unity) themselves are very helpful and have provided an extensive documentation both hosted online, and offline when you download Unity. They also host their own resources section where you can download tutorials, assets (such as the awesome "Gem Shader"), Scripts, and more. Unity's Customer Support is breathtaking.
Unity3D updates fairly often. Buying Unity 2.x allows you to freely upgrade to any Unity 2.x version in the future. You will only have to pay again with the onset of Unity 3.x. Each update, though small increments in version number, contain plethoras of new features that make your experience a better one. If you own Unity Indie ($200), you can update to Unity Pro ($1, 500) for $1, 300 (the difference between indie and pro). Unity Pro gives you features like deployment for Windows, lots of new graphical effects, realtime shadows, and more.
I can't stress enough now how much you need Unity if you make games and use the Macintosh. It gives you the freedom, as I've heard said, to go from the drawing board to a working prototype within a week. Certainly I can confirm that this is true.