Well, let's see...
1. It's a break from the usual "save the princess" plot.
2. Luigi has the starring role.
3. Instead of the Mushroom Kingdom, you actually go to Earth cities.
4. It's easy!
"ooOOooooOOOooo, it's just some stupid edutainment game! Who cares about it?"
Well, I do!
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Go ahead and hate your neighbor, go ahead and cheat a friend. Do it in the name of heaven, you can justify it in the end. There won''t be any trumpets blowing come the judgement day. On the bloody morning after, one tin soldier rides away..