I'm unsure about forcing user to do a tour. There are plenty of other methods to improve discoverability.
However there is no doubt that tours are valuable. If you are doing it I would suggest a design like Twitter's. This is a 60 second tour for all new accounts.
The design of the tour itself is also excellent. It provides good progress information and encourages users in a "learning by doing" fashion. I would highly recommend taking a look at it (you need to set up a new twitter account).
If you do feel it is necessary to force a guided tour then make sure it is extremely short with plenty of obvious skip options. The last thing you want to do is to annoy your users this early in the relationship!
Personally I would prefer a tour which is optional for the user ... perhaps one that appears as notification type message when a user is new that can be clicked away (don't show this again). This is a much less forceful approach. However I am sure that Twitter and the gaming industry have done the research. So it may well be worth following the trend.
Another idea is to make use of the progress bar model employed by Linkedin and Wikipedia... where you have some notion of completeness to become a "good" user. I think think this can encourage people to explore parts of the system that they haven't.
Gamification methods can be employed in other ways. For instance here on UX stack exchange you can gain a badge for visiting every page of the FAQ. A clever way to ensure that users read some of the documentation.
A recent question here on UX stack exchange suggested flashing up advanced search options or hidden menus... to expose functionality. I like this idea and think if carefully designed it could be very powerful for increasing discoverability.
There are plenty of creative ways to increase discoverability. The key to keep in mind in this sort of design is to be polite to your user. Remember they will have many demands on their time and many different contexts of use. They may well just want to get on with the job. So don't force them down a particular path. Give them options and plenty of ways to return to learning as their use of the product grows.