  AGNs with narrow Balmer lines show various extreme properties. In particular,
rapid X-ray variability, steep X-ray spectra, peculiar optical and UV line
ratios, and possibly peculiar line profiles. Since all these phenomena occur
together they are likely to be related to one specific underlying physical
parameter. I review recent evidence, based on HST imaging of low z quasars,
which suggests that the H-beta line width and continuum luminosity of quasars
provide a reasonably accurate estimate of the black hole mass. This implies
that narrow-line AGN have relatively low black hole masses, and thus high
L/L_Edd, as independently suggested based on their steep X-ray spectra. I
present additional evidence suggesting that the X-ray variability and the radio
loudness are primarily driven by the black hole mass. The high mass inflow rate
into the core of narrow-line AGNs may produce a denser and more enriched BLR, a
high column radiation pressure driven outflow, and a smaller illumination angle
for the NLR, as suggested by the observed emission line properties. Narrow-line
AGNs may thus provide important clues for understanding the rich overall
phenomenology of AGNs.
