Alternating helps the tedium a lot. Play an Imp to 50, then a Pub to 50. Gives you more time to forget a planet quest before you have to go do it again on another alt.
Although so far I've been enjoying my dual-leveling a great deal - I have a vanguard and an operative that I'm keeping at the same level, so I'll play Hoth as Republic and then Hoth as Imperial, and it lets me do a sort of side-by-side comparison of the two stories and see how they fit together and get a sense of the overall storyline. It does make it easy to lose track of the class storylines, though.
So true. As I'm playing all the 8 classes side by side, with a concentration of one over the others as they hit level cap, I've come back to a storyline and gone "What? Huh? Where was I?"