You're a Great Guy, But...
But - the people that dwells in the land is powerful... (Num. 13:28)
On the surface it appears that the spies did nothing wrong in describing what they had seen while scouting the promised land. They had been sent to make their own observations and to report whether the land was good or bad and if the people living there were strong or weak. In fact, that is exactly what they reported - they relayed the truth as they had perceived it. What then was their sin?
Imagine you receive a phone call asking for information about a friend for a shidduch, a prospective date. The boy is very bright, refined, has good social skills and comes from a broken home. Your phraseology in your response will make or break the shidduch. One response might be: "The boy is extremely intelligent, very kind, and sociable ...but he comes from a broken home." You are implying that his negative qualities outweigh all of his good attributes. The word "but" negates all that was previously stated.
Although the spies gave over the report exactly as they had seen it, the fact that they mentioned the word "but" revealed that they had a personal agenda. If their report was purely factual there would be no need for such a qualifier; they should have continued to state the bare facts. By using a word that implied a contradiction to the optimism of their first two sentences, they were, in effect, telling the nation that no matter how rich and blessed the land was, it was beyond their reach; the inhabitants were just too strong and their cities too invincible. It was the way the spies phrased their report that brought about their punishment. (Ramban)
One can apply this principle when complimenting someone. If one says, "The steak was unbelievable, but it tasted a bit raw," or "You're a great guy, but you are annoying," he has negated the entire compliment, implying that the positive is irrelevant in comparison to the negative. Never put a compliment and the word but in the same sentence.