T. S. Eliot praises Donne's ability to unify the intellectual thought and sensation of feeling. Why? T. S. Eliot's "dissociation of sensiblity"...

I haven't read T.S. Eliot's essay "The Metaphysical Poets" (1921) in quite a long time and had to review it briefly to make sure I wasn't completely off base, but I recall Eliot's argument to be that Donne's poetry reflects how things were for the poet before the supposed "dissociation of sensibility" set in.

Eliot explicitly contrasts Donne to later, Victorian-era poets: "Tennyson and Browning are poets, and they think; but they do not feel their thought as immediately as the odour of a rose. A thought to Donne was an experience; it modified his sensibility." He goes in the next paragraph to develop what he calls the "theory" of "a dissociation of sensibility":

We may express the difference by the following theory: The poets of the seventeenth century, the successors of the dramatists of the sixteenth, possessed a mechanism of sensibility which could devour any kind of experience. They are simple, artificial, difficult, or fantastic, as their predecessors were; no less nor more than Dante, Guido Cavalcanti, Guinicelli, or Cino. In the seventeenth century a dissociation of sensibility set in, from which we have never recovered; and this dissociation, as is natural, was aggravated by the influence of the two most powerful poets of the century, Milton and Dryden. Each of these men performed certain poetic functions so magnificently well that the magnitude of the effect concealed the absence of others. The language went on and in some respects improved; the best verse of Collins, Gray, Johnson, and even Goldsmith satisfies some of our fastidious demands better than that of Donne or Marvell or King. But while the language became more refined, the feeling became more crude. The feeling, the sensibility, expressed in the Country Churchyard (to say nothing of Tennyson and Browning) is cruder than that in theCoy Mistress.

So, to answer your question (I hope), Eliot praises Donne and the metaphysical poets in general and finds a deficit, if we can call it that, in the more modern poets. I've always understood Eliot to be talking here more about himself and his modernist contemporaries than he is about Victorian poetry.