  The 2014-2015 prediction, discovery, and confirmation of record high
temperature superconductivity above 200K in H$_3$S, followed by the 2018
extension to superconductivity in the 250-280K range in lanthanum hydride,
marks a new era in the longstanding quest for room temperature
superconductivity: quest achieved, at the cost of supplying 1.5-2 megabars of
pressure. Predictions of numerous high temperature superconducting metal
hydrides $XH_n$ ($X$=metal) have appeared, but are providing limited
understanding of what drives the high transition temperature T$_c$, or what
limits T$_c$. We apply an opportunistic atomic decomposition of the coupling
function to show, first, that the $X$ atom provides coupling strength as
commonly calculated, but is it irrelevant for superconductivity; in fact, it is
important for analysis that its contribution is neglected. Five $X$H$_n$
compounds, predicted to have T$_c$ in the 150-300K range, are analyzed
consistently for their relevant properties, revealing some aspects that
confront conventional wisdom. A phonon frequency -- critical temperature
($\omega_2$-T$_c$) phase diagram is obtained that reveals a common phase
instability limiting T$_c$ at the {\it low pressure} range of each compound.
The hydrogen scattering strength is identified and found to differ strongly
over the hydrides. A quantity directly proportional to T$_c$ in these hydrides
is identified.
