Not sure about 'most' MMO's, but I know EQ only offers the tutorial as a Demo. All the demo's I have played usually leave you wanting more, which is exactly what their supposed to do. The fact that people are asking for more in the demo tells me it is working as intended.
WoW offers a 14-day trial period. A person can get to see a lot during that period of time. Not quite sure about other games that aren't F2P.