Pemenon,
You've got the right idea about positioning yourself. The package will be your stats (GMAT, GPA, ECs, work experience) plus 3-5 qualities that highlight your strengths.
I think highlighting things that aren't common to engineers is very important. What are the engineer stereotypes? Academic, bookish, poor interpersonal skills, etc. Highlight qualities that bust these stereotypes.
One of the keys to coming up with your list of qualities is come up with good stories to support your claims. You say "creative thinker", why do you say that? Do you have a good story you can tell in 250 words (essay) or 2-3 min (interview)?
I would take your list, come up with examples, then narrow it down to the best 3 to 5. The key is to be holistic about your positioning. Like bsd_lover said, you should be able provide a well-rounded summary of your position in as few words as possible.
Concerning sociability, I actually think it's a worthwhile quality to highlight in your essays, especially for an engineer. However, you can't just say "I'm social", it has to be deeper and more relevant than that. Come up with an story where the reader will think "Wow, this guy will fit in great here" (that is where sociability fits in).
RF