The crazy little game that keeps on giving.
Competition was really emphasised in Nations, countries versus countries was the main aim, but also individual ranking, based on your multiplayer races. United takes this further, and puts the entirety of the solo maps into a world ladder system. You can play offline, set a training time, and get a medal. But when you have practiced enough you can choose to enter official mode, to submit your time to the ladder system. Skill points are given based on your relative level compared with your time against others, these points are used to rank you. Coppers (in game currency) are given for beating the official gold, silver and bronze times on the standard tracks. The first time you race officially you do so without charge, however from then on your are charged coppers to submit a time. You should gain enough coppers to retry officially several included tracks over and over many times. But should you run out you can wait until the next day, as you are given free coppers each day just for logging in. What this does is make the solo aspect of the game competitive. You aren't playing directly with others, you have no time restrictions, you can just set an official time at your leisure and watch as your rank climbs.
You can test out your creative skills and make some tracks of your own, presented in a quite intuitive 3D format you set about laying blocks of track, you can upload these tracks and have people all over the world playing them. By uploading them you can gain more coppers to buy some other tracks or to try to set better times and then increase your rank further. The idea is to keep the cycle of content, keep people making tracks for coppers, and keep tracks being made for people to play. You can download these track, and they will cost you some more coppers, and then you can race on them any time you wish. The system seems to work well and encourages players and mappers alike to participate. Many of the user tracks are also very good, they range in difficulty, but a little voting system seems to keep the standard fairly good. The best feature of this game is just the huge numbers of tracks at your disposal, near infinite amount of tracks, and how they are all add to the experience of just fun racing.