Compared to standard it has LOD, terrain and a resolution of 1024x768. Terrain can be done with models, too, but the 800x600 resolution of the standard edition seems pretty low to me. A lot of people recommend to buy the commercial edition as you get the most out of your money with it. Personally I use the Extra edition, but I'm not a professional developper and I bought it when it was cheaper than it is now. The commercial edition has some nice features like dynamic shadows, the ability to play movies, multiplayer, bones, and more which the Extra has not.

It's all a question of what you want to do with it and how much money you are willing to spend.