I'm a self-admitted alt-o-holic. It finally dawned on me why I'm getting burned out. Put simply, it is always the same. I hate to do it, but I will use WoW as an example of doing it right.
In WoW;
First: Create the character. You begin in your race's starting point. (Humans start in Northshire). Then you can go to Elwyn Forest, then Westfall, then Redridge, etc. OR, you can choose to go somewhere else, like visit the dwarves or the elves. If you get bored with one area, there is somewhere else you can go. To a limited degree it still applies to BC, WotLK, and Cata. They all had a few zones of equivalent levels you could choose from.
With SWToR, you can not! The quest/planet path is laid out. <EDIT>With the exception of the starting planet,</EDIT> everyone in a faction visits the same planets in the same order, every time. Yes, there are some very welcome class detours that pop-up, but they are just one quest and then you are back to the same grind.
The handful of class-specific quests on each planet are good, but to continue to level up, you have to keep running the same old quests, repeatedly. I mean, how many times do I have to recover the rak-goul serum for those people?
Unfortunately, there is no easy solution to this. It would require a lot more planets and a lot more quests to add variety and spice to the leveling process.