What is "Bokeh"?
When you use a lens at a wide aperture (ƒ1 - ƒ5.6), 'Bokeh' refers to the out of focus portions of the image - in particular its visually pleasing nature or lack thereof. Yes, it's YMMV and completely subjective!
Things to look for? An absence of "double-vertical" lines, perfectly circular highlights and a gentle gradation between the in-focus region and OOF parts.
From experience, Bokeh is more pleasing if there are a large enough number of lens diaphragm blades to form a circular (rather than pentagonal or hexagonal) aperture opening. Thus lenses with eight or more blades will usually give a more pleasing out-of-focus effect than those with only five. (N.B. This partially explains why Voigtländer 35mm, 50mm and 75mm LTM lenses feature ten diaphragm blades, whilst the 50mm Summilux-M has twelve.)
Alongside this, the lens optical design also has an effect, in particular how it corrects for spherical aberation in the forward and rear out of focus regions.
Links & Articles
The following URLs explore these issues to the nth degree:
For the technically minded, there is also an excellent article written by Harold M. Merklinger and published in Photo Techniques, May/June 1997. It's available online in Adobe PDF format and can be downloaded from the following URL (340 KB):