What makes a good showreel? What am I looking to achieve?

Since I feel I'm lacking in footage for a showreel I should take into account what makes a good show reel and keep it in mind when working on future projects. This will hopefully make me aim for at least of a few seconds of quality work in projects that I will be proud to represent my skills and ideas.

I see myself as a generalist so would want to illustrate a range of skills, though I am aware that I would probably have to create and change my showreel dependent on what I am applying for. Aesthetically I would like something that's quite striking and illustrates my interest in cinematography and strong imagery, such as below.

vfx/design reel 2012 from Tim Borgmann on Vimeo.

However I wouldn't want it to be solely focused on VFX and would like to also illustrate my interest in narrative and story based interested. Perhaps it is worth considering creating two seperate show reels to illustrate this, or at least have some form of theme or tone that follows through, such as below. I'm aware I wouldn't be able to achieve something with such range and scope since this is cinema work.

MPC Film Reel 2013 from MPC on Vimeo.