Well, I also pre-ordered on day 1, because I thought to myself, "How long did it take me to go from Voss (level 44) to Corellia (Level 50)? Is that amount of content worth $10? Decided that yes it was. Pre-ordered.
Each level takes longer to reach than the last, so I figure Makeb will be at least as big as Voss combined with Correlia, or will provide enough content to fill in the five levels. Whatever form that content takes, 5 levels of anything, is worth $10 to me.
I should also point out however that it is ridiculously easy to entertain me. And when Star Wars is involved its impossible for me not to be entertained. Stick Star Wars in the title and I'll play it at least once.
For example, I never bothered purchasing Angry Birds until they released the Star Wars version. The second they did though, I bought it. It was fun enough to play it through to the end, but I've still never bothered with the original. Likely never will.