A full answer to the question requires determining the constituent elements of preaching. And for someone to be unreservedly characterized as a good preacher, he must be excellent in all those constituent elements, although it is probably rare to find someone who is equally outstanding in all areas. Obviously the content of preaching, the thing preached, is a major constituent element. No amount of excellence in other areas will make up for bad content (though it may be partial compensation for slim content); and excellence in this area can make other deficiencies to be overlooked or less noticed. The manner of preaching, the actual delivery and sound of the message are also major influences. Since preaching involves speaking, there are mechanical questions of voice and volume and articulation; since it involves presenting content there are questions of holding attention and organization of auditory material. If people can't hear or can't follow the content being presented, it's unlikely they'll immediately leap to the conclusion that a good preacher has been among them. But these questions allow for a certain flexibility, as there are styles of speaking that are highly valued in one culture and earn disdain in another. The context of preaching also enters into it: the demeanor of the preacher, if his actions undercut the force of his message, things of that nature. It's probably logically distinguishable from the actual quality of preaching, but in practice it's bound to color people's estimation. You have also the calling and character of the preacher to consider. A clear calling and a noble character may well cause defects in other areas to be tolerated for a long time; but I think without good content and amenable delivery, it will still be difficult for people to be really enthusiastic about the preaching. Of course our own ignorant prejudices, idiosyncratic preferences, and irrelevant partialities may make it difficult to give an accurate estimation of good preaching in reference to particular persons; so ultimately the safest ground is probably to say that a good preacher is one who has built with gold, silver, and precious stones upon the foundation of Christ. But what I mean when I say that someone is a good preacher, is that he is pleasant to listen to in the process of saying true and interesting things on a cohesive topic, while showing their derivation from Scripture.