Welfare reform. By not doing completely away with welfare reform, we showed that incrementally moving the program towards more responsibility on the part of the recipients would not result in millions of more homeless, etc.
Tax Cuts. We constantly demonstrate that tax cuts work by showing that each time we do a cut, revenues increase. But we could not repeal the income tax and replace it without those examples to prove our trustworthiness in handling the tax code.
Welfare reform came when the GOP controlled the House only and compromise was the only way to accomplish anything at all. Still, we won that one by holding out for the most we could get, not by conciliatory gestures.
Tax cuts were the product of compromise, true. This compromise also producted unprecedented spending. Did the ball advance in our direction? No, it advanced quite a bit in the other direction - adding debt is not a consequenceless nor neutral effect. Tax cuts are only meaningful in the context of commensurate spending cuts.
In the context of a GOP that controls the House, Senate, and Presidency, compromise comes with an immense opportunity cost - our time to act is frittered away, giving the opposition the ability to neutralize the advantage.