There are of course missions in this game that appear in a certain order. You need to do a mission to unlock subsequent missions. But the missions are not the ONLY point of playing San Andreas. There are many other things that are required to reach a full 100% completion. You can see the progress you're making in the menu screen, under achivements.

The things needed to get a full 100% include:

* Finishing all missions
* Taking over all territories
* Finding all 50 horseshoes
* Finding all 50 oyster shells
* Tagging all 100 grafitti spots
* Getting max on ambulance missions
* Getting max on vigilante missions
* Getting max on firetruck missions
* getting max on taxi missions
* valet parking
* shooting range
* special photographs

When we finished the main mission track and had all territories under our control (plus a few other items done) we were at 73%.

Grand Theft Auto San Andreas Walkthrough