It can be argued (pretty easily in fact) that harmony is the most important aspect of music. Melody, rhythm, and texture are extremely important but there would be no meaningful melodies without harmonic back-drop, rhythm wouldn't mean anything since chords would not be changing, and texture would remain a static haze.
Fortunately, Tonal Harmony is an extremely expansive subject and you can really never learn to utilize all of it correctly (although many folks of quite young ages seem to think they know an awful lot of harmony). Around 1908, classical music reached a legitimate crisis where chromatic harmony had stretched the dominant-tonic relationship so severely that the only options other than regression were things such as atonality and pan-tonality. Being very honest, however, popular music has not really left the 1700s in terms of harmonic advancements!
The key to tonal harmony is the dominant-tonic relationship, and most people, even fans and advocates of atonal classical music like myself, agree that the most "pleasing-to-the-ear" music is tonal. Thus, for popular music, it would be rather odd to employ excessive passages of atonality since the goal is popular appeal. Why not just write classical music if you really like atonality? For some reason, however, and this is extremely obvious from examining the multitude of chord progressions that are available on this wonderful and valuable website, most musicians still use a direct means of harmonic progression.
What I am saying is that most current (and past) popular music literally goes I - IV (or ii) - V - I for major lengths of time. Of course, most songs have some chromaticism in them, such as borrowed chords (using chords obtained from the harmonization of scales on the similar tonic as the one you are using - for example: Ab Major chord in C Major), but the majority of sequences/progressions are in this striking literal character!
I seem to suspect that musicians are weary of using chromaticism in their works because they feel it weakens the dominant-tonic relationship. But this is not true. All major treatises on Tonal Harmony (including Piston & Kostka/Payne) make the point that well-crafted chromaticism strengthens the final cadences (V - I authentic cadences). This includes things such as secondary dominants, borrowed chords, and true chromatic chords like Augmented Sixths and Raised/Lowered Fifth Dominants.
I have searched hard, just for curiosity, to see if any bands use things like I mentioned above. So I took to giving an open minded examination to the chord charts of many supposedly "challenging" songs that were compiled here. So far, the most exotic thing I have found is the use of the Neapolitan Sixth chord as a true subdominant chord in "Dawn Is A Feeling" by The Moody Blues. Now I am sure there are other isolated examples, but it is strange how severely isolated they are, because those chromatic progressions are mind-blowing in terms of their emotional power. For example, some of the more chromatic music is Motown, and we all can agree on how powerful those songs are!
So, my challenge to my fellow popular music composers is to incorporate (tastefully) one element of "advanced" chromatic harmony into your next work. Obviously don't just stick it in... do some research into things like Augmented Sixths and Secondary Function Chains, and work hard until you discover, like I have, how it can make your compositions come to life.