  Emission lines from metals offer one of the most promising ways to detect the
elusive warm-hot intergalactic medium (WHIM; 10^5 K<T<10^7 K), which is thought
to contain a substantial fraction of the baryons in the low-redshift Universe.
We present predictions for the soft X-ray line emission from the WHIM using a
subset of cosmological simulations from the OverWhelmingly Large Simulations
(OWLS) project. We use the OWLS models to test the dependence of the predicted
emission on a range of physical prescriptions, such as cosmology, gas cooling
and feedback from star formation and accreting black holes. Provided that
metal-line cooling is taken into account, the models give surprisingly similar
results, indicating that the predictions are robust. Soft X-ray lines trace the
hotter part of the WHIM (T>10^6 K). We find that the OVIII 18.97A is the
strongest emission line, with a predicted maximum surface brightness of ~10^2
photon/s/cm^2/sr, but a number of other lines are only slightly weaker. All
lines show a strong correlation between the intensity of the observed flux and
the density and metallicity of the gas responsible for the emission. On the
other hand, the potentially detectable emission consistently corresponds to the
temperature at which the emissivity of the electronic transition peaks. The
emission traces neither the baryonic nor the metal mass. In particular, the
emission that is potentially detectable with proposed missions, traces
overdense (rho>10^2rho_mean) and metal-rich (Z>0.1Z_sun) gas in and around
galaxies and groups. While soft X-ray line emission is therefore not a
promising route to close the baryon budget, it does offer the exciting
possibility to image the gas accreting onto and flowing out of galaxies.
