As a Republic player I began to notice this around level 19-20 when I finished the last mission on Corusant. I then went to Taris and was a little surprised to already seem above that planets level range (16-20). I felt that I had to avoid getting too much XP and skipped about half of the content on that planet. There were even a couple of guys who asked in the general chat "What are all you level 21+ guys doing here? Why dont you move on?"
I am now level 25 and not even half way thru Nar Shaddaa, and have recently realised that most of my missions have been green since level 15. I quess it is just because there is so much fun stuff to do, I've been thru a couple of flashpoints, did my fair share of duty to the Republic in warzones, and tried out the space combat as well. (Which I found too give a surprisingly generous amount of XP I might add - a little too much perhaps) Crafted a lot too!
I am also the kind of person who wants to do all the missions in an area before moving on, so I definately wouldn't mind if they turned down the xp gain a little.