Existing observations of the cosmic expansion history place strong restrictions on the rate of large scale structure growth predicted by various dark energy models. In the simplest Lambda CDM scenario, current observations enable percent-level predictions of growth, which can be interpreted in terms of the expected abundance of massive galaxy clusters at high redshift. I will show that these predictions from current data set a firm upper limit on the cluster abundance in the more general class of quintessence models where dark energy is a canonical, minimally-coupled scalar field. While the most massive clusters known today appear to lie just below this limit, future discoveries may reveal more anomalous clusters that would rule out not only the standard cosmological paradigm but also many of the best-studied alternative explanations for cosmic acceleration.