You need to do like 95% of the quests on each planet inc bonus series, do flash points, pvp warzones and space combat missions or you will fall behind sadly.
no offense, this is a complete bullcrap. i am lvl 42 halfway to 43 on Hoth and almost finished it, already 3 levels above the planet (most mobs are lvl 39). never did bonus series on any planet, skipped 2 planets doing only class quest, did only half of Tatooine, havent grind space missions - occasionally dailies only, skipped some flash points, other did only once. Did plenty of PvP, true, but still, i think if one really does everything you say he will find himself way above leveling curve, and by way above i mean gray planets one by one.
this game offers most comfortable leveling i ever witnessed in an mmo yet.
it has flaws, but leveling is not the one. in fact, it is one of the strongest and well done part of the game.