The imposition of power by the majority ethnic group (or simply the ruling ethnic group) is the direct cause of political discord, as it forces individuals within other ethnic groups to follow suit and band together politically. Furthermore, in situations where a dogma (usually religious) is adopted by an ethnic group, such as the Sunni Muslims we now see rising in Iraq against the Shia Muslims that are in control of the nation, one ethnic group seeks to assert power and authority over another, which was directly caused by the Shia Muslim Government's imposition of power onto the Sunni Muslims in the first place. Ergo, the imposition of power of a majority ethnic group onto another ethnic group is the cause of intrastate turmoil. In nations where dogma isn't applied or persecution isn't an instrument of the state, this manifests itself through the imposition of economic power of one ethnic group onto another.
I'm not discrediting the solution of removing ethnic groups to solve the issue (though it would displace the poor souls who were born into a nation while simultaneously being born into an ethnic group that is confined due to ethnic-economic instability), but I am stating that the cause of the problems we see in today's claimed heterogeneous societies are the results of ethnic favoritism. There are many feasible solutions for the issue, but they do not usually agree.
By the way, apologies for the time this response took. My left earbud died and set me on the course to finding another pair of earphones, which then led me to realize that the other pair of earphones has a dead right earbud, so I am now listening to music through two different earphones via a splitter.