There are many different aspects to final cut that make it much more advance than iMovie. iMovie was the editing software I used for my AS production. FCE is considered a step above iMovie in terms of it capabilities. FCE allows the user to do a range of effects such as layering clips and special effects which are beyond what iMovie is capable of.
FCE is a better system but is harder to get the hang of, and requires a greater understanding of the technology used. iMovie is easier to work with but does not have the best effects, but is better for beginner editors such as at AS.
The layering tool in FCE is particularly effective as it can place one clip layered over another. By changing the opacity of the clips both clips can be played together over one another. During the editing of our rendition of Threw It On The Ground I included repeat looping in the editing. This is when a section of a clip (usually around a second long) is repeated four or five times to create an effect of a clip being more in time to the beat of the song. For the song we remade this worked really well, as the repeat clip was mtahced well to the electronic beat to the song.
Exporting your finished video on FCE is also much more difficult than in iMovie. The video must be exported with specific adjustments and if not followed absolutely precisely than the video cannot be exported properly. This has been a problem so far this year, but I hope to improve this by learning to export more as the year progresses.