  Since the discovery of high-$T_c$ cuprates the quest for new superconductors
has shifted toward more anisotropic, strongly correlated materials with lower
carrier densities and competing magnetic and charge density wave orders. While
these materials features enhance superconducting correlations, they also result
in serious problems for applications at liquid nitrogen (and higher)
temperatures and strong magnetic fields, so that such conventional
characteristics as the critical temperature $T_c$ and the upper critical field
$H_{c2}$ are no longer the main parameters of merit. This happens because of
strong fluctuations of the order parameter, thermally-activated hopping of
pinned vortices and electromagnetic granularity, as has been established after
extensive investigations of cuprates and Fe-based superconductors. In this
paper I give an overview of these mechanisms crucial for power and magnet
applications and discuss the materials restrictions which have to be satisfied
in order to make superconductors useful at high temperatures and magnetic
fields. These restrictions become more and more essential at higher
temperatures and magnetic fields, particularly for the yet-to-be-discovered
superconductors operating at room temperatures. In this case the performance of
superconductors will be limited by destructive fluctuations of the order
parameter so higher superfluid density and weaker electronic anisotropy which
reduce these fluctuations can become far more important than higher $T_c$.
