  We performed a search for WLQs in the spectroscopic data from the Sloan
Digital Sky Survey Data Release 7 based on Kohonen self-organising maps for
nearly 10^5 quasar spectra. The final sample consists of 365 quasars and
includes in particular a subsample of 46 WLQs with equivalent widths W(MgII) <
11 A and W(CIV) < 4.8 A. We compared various properties of the WLQs with those
of control samples of ordinary quasars. Particular attention was paid to
selection effects. The WLQs have, on average, significantly higher
luminosities, Eddington ratios, and accretion rates. About half of the excess
comes from a selection bias, but an intrinsic excess remains probably caused
primarily by higher accretion rates. The spectral energy distribution shows a
bluer continuum at rest-frame wavelengths > 1500 A. The variability in the
optical and UV is relatively low, even taking the variability-luminosity
anti-correlation into account. The percentage of radio detected quasars and of
core-dominant radio sources is significantly higher than for the control
sample, whereas the mean radio-loudness is lower. The properties of our WLQ
sample can be consistently understood assuming that it consists of a mix of
quasars at the beginning of a stage of increased accretion activity and of
beamed radio-quiet quasars. (Abstract modified to match the arXiv format)
