I don't agree. Die Hard is a landmark in the genre - a hugely influential action thriller that cast a shadow over many others in its wake. The Christmas debate is also a red herring that detracts from this fact. You could argue the plot is flawed, but so is the plot of Casablanca (grossly, in fact, as there was no wartime escape route out of Casablanca). A great plot isn't necessarily unflawed, and Die Hard and Casablanca both have great plots.

My full thoughts on the matter are here, if you are interested. :)