What's the problem with consolidation of power?

[American]

"What has destroyed liberty and the rights of man in every government which has ever existed under the sun? The generalizing and concentrating all cares and powers into one body, no matter whether of the autocrats of Russia or France, or of the aristocrats of a Venetian Senate. And I do believe that if the Almighty has not decreed that man shall never be free (and it is blasphemy to believe it), that the secret will be found to be in the making himself the depository of the powers respecting himself, so far as he is competent to them, and delegating only what is beyond his competence by a synthetical process, to higher and higher orders of functionaries, so as to trust fewer and fewer powers in proportion as the trustees become more and more oligarchical." --Thomas Jefferson to Joseph C. Cabell, 1816. ME 14:421

Well I think Jefferson explains it quite well here. Man cannot be trusted with much power (because of inherent character flaws), so it must be diluted to the maximum extent possible.

Nearly all men can stand adversity, but if you want to test a man’s character, give him power. -- Abraham Lincoln