Your reel needs to be short, concise, and attention getting in the beginning. Don't include animations of tunnels, space ships, fractal planets, and so on. These have been done to death. However, if you have an animation that is particularly spectacular on one of these themes, then included it- but flying a camera down a round tunnel is not very original. Think about your reel.
Content is very important of course, but so is presentation.
Speaking of content, it very important that your reel have some. Well, most movies today have great effects, but are seriously lacking on the content end. Not that eye candy is bad, but it's mostly the computer doing the work. Trust me, after a time, you'll be able to recognize what software does what effect. Someone reviewing your reel is not going to be impressed that you made a KPT Bryce landscape, but they may be impressed with what you decided to do with it.