Cris, you said you found bonus missions on Nar Shaddaa, when you're 24. You don't start the bonus series until after you finish the planet. Taris Bonus starts around 20(you can see the questgiver in the starport), and Nar Shaddaa's around 29 or 30. They're an extra set of chains and hubs that are higher level that unlock once you finish a world.
But I assume you meant the bonus quests that are attached most of the game's quests that basically put the grind in those optional bonuses.
Sorry, nomenclature bugged me a little. :P
"Sir,we are beginning our search for these rumored 'underwater villages'.They will not stay hidden for long". -Battle Droid Commander OOM-9!