Every time someone asks a candidate for licensure, "Tell me why dispensationalism is wrong," in the candidates and credentials committee of my presbytery I cringe.
I'm not a dispensationalist, but the answers sought and given are often caricatures rather than fair assessments.
Some years ago I sought to explain to one of the last dinosaurs of dispensationalism why dispensationalists couldn't be true Calvinists. The dispensationalist I was talking to was John Sailhamer. The setting of our talk was a breakfast at the 2000 meeting of the Evangelical Society in Boston--where John was assuming the presidency of ETS. The issues we discussed were Amyraldianism and the order of God's decrees (infralapsarianism vs. supralapsarianism). And what I learned was that I didn't know Reformed theology as well as my dispensationalist friend, let alone dispensationalism.
By the end of our conversation most of my cocksure views had been gently removed and I promised myself I would never again presume to define dispensationalism's faults--at least to a real live thinking dispensationalist. I don't agree with dispensationalism, but I'm not an expert on it. And when I did meet an expert, I found certain stock Reformed criticisms of dispensationalism rather threadbare. Dispensationalists deserve to be taken seriously. Reformed believers don't appreciate being accused of the things Arminians suggest against us. Most are false. We need to be as careful in what we say about dispensationalism as we ask Arminians to be toward us.
For more on illegitimate criticisms of dispensationalism read this post by Dan Phillips.