Because the artists are drawn from the pool the CD cover must represent a variety of music.
The cover is for a compilation, hence something nondescript in terms of artist or music genre is appropriate. Symbol,patterns, textures are good choices. Not to dark (in terms of mood) as this may not be fitting in terms of the tracks on the compilation. By that same token not to bright which could be construed as overly cheery, or playful. Again it may not be indicative of the contents of the compilation
The key is to look for something universal. An opportunity to look at common ( boring I know ) graphic elements. Techniques which have become overused, but I think work well because they cannot be associated with one type of music. e.g.
Use of generic brushes, starburst background To create something that is ok. Not awe inspiring, but is easy on the eye, makes no assumption? about the type the music. Simple, clean
When something has been used a lot it is understood, sometimes expected and welcomed. Again these are just my thoughts.
Jacob Cass has written a nice article  about these repeated trends in current graphic. The prevailing view is that graphic design is currently stiffled ( at the time of writing ) due to the overuse of stock imagery, and whilst I understand that his point of view I also think
If it works, go with it !!
Other forms of inpiration
I also looked at other form of promotional material. The catalyst for this being Glastenbury. Current generations even though they may have their preference are not polarised are free to enjoy the experience of diverse musical expressions more openly. Staunch adherences to a single genre such as Mods, Rockers are less prevalent today.
As such festivals / compilations represent diverse acts. Festivals once the preserve of indie groups are now open more commercial sounds to the pleasure of some, the consternation of others. Due to its inclusivity I felt it was worth a look at past promotional material
As you can see here all the common elements are present, producing a pleasant enough design, yet generic enough to have that has a far reaching appeal. serves its purpose
Theses flyer demonstrate how entrenched these design elements have become. My take is I like them. Tm guessing people like them or at least dont mind or they would not be so prominent.
I guess the trick here is to try to use these element in an original way. All of them with varied look and feel.
As a budding graphic designer these techniques are well worth knowing
Glastenbury Promotional Material
Jimmy Tirrell, in his this 2009 campaign achieves a universal feel by combining part of multiple artist to complete the whole, with use of halftone. The third makes use of the cassette, again a choose to provide that musical reference without attached itself to a particular genre.
We all know what it is, but it is hard to argue with its appeal. I like it as much now as when it was run. Highly successful, yet has all the traits of generic design i.e the silhouette figure and device, the injection bright colour which grabs the attention. screams youthfulness etc.
It works so well because how generic it is. The girl could be anyone listening to any music genre, hence has far reaching appeal. The main point is that she is enjoying herself as can be seen from her body language and its all due to here clearly detectable ipod.
As fresh as it looks this is an old campaign, and whilst these elements are commonplace today chances are it originates here. I would be unlike Apple to rehash something done before, and lets face it they have pretty much been the trend setters in many aspects of design for the last decade.
Even though they have been over used they have the benefits of generic and have universal appeal. All things which compilation cover must portray.