A lot of good points (and a few good jokes) have been made, but here's my rambling thoughts on wound/plain Gs, just for the hell of it:
For any given gauge, say .18, for example, a wound string and plain string are about as different as can be. Since the wound string is two-piece, the inner, plain core wire is pretty thing gauge and so is the outer wound wrap. The winding wrap is usually very fine gauge and somewhat delicate, compared to the lower 3 strings' larger-gauge windings. Over time, the pressing and rubbing against the frets will dent and grind away the bottom of the wound wrap (everybody's probably seen this when changing old strings). This is probably the biggest factor in strings going "dead" over time. Plain strings don't seem to really ever start sounding dull, but I always figured that, once the wound strings have a lot of wear along the fretting points, they become somewhat segmented. Though the inner core wire is still what handles the tension and vibrates, I imagine the segmented nature of a worn-down winding kills a bit of the transfer of vibration along the string's length.
The wound G's windings are so delicate that, if you bend a lot, sometimes the dent/gouge at the fretting points will get to the point that it actually wears through and severs the wound outer wrap. The inner core wire is also probably about as thin as the high E, so it's easier to break than a plain G of the same thickness as the overall combo of inner and outer wire on a same gauge wound string.
Loudness and intonation: For the sake of both factors, it's best to consider the wound strings as more or less the gauge that their inner wires are.
The thicker a wire you have, the longer the string length gets to intonate correctly, with respect to smaller strings. That's why archtop bridges get increasing shorter (string length wise) from low E to G, then jump back longer for the B and then shorter again for the Hi E. That's b/c most are designed for a wound G set of strings and the wound G is about as thin (on the inside) as the Hi E. Without being able to adjust the individual string lengths on a carved wood bridge, a plain G will usually fall sharp w/ respect to the other strings b/c it's actually thicker than the B.
Loudness is the same story, the plain G is usually about the thickest string on the guitar (vs. the inner wire in the wound strings). As such, it's the loudest as far as magnetic pickups picking up the vibrations. With non-adjustable pickups, like Strat pickups, etc, this can be a problem w/ relative string balance. The stagger on Strat pickup pole pieces is designed for wound-G strings, and makes the G pole the tallest, in order to balance its slight volume against the louder/thicker wound strings and also the louder/thicker B and E. Throwing a plain G on will make for a string that's pretty loud against its neighbors.
Neither of those factors matter with individually adjustable poles or saddles, but older guitars are usually designed for the wound G and fixed adjustments on them are preset accordingly. Likewise, some modern parts are designed that way as it's more or less traditional.
As for bending, the thing that throws me off is that it seems you have to physically bend the G farther (further across the fretboard) w/ a wound string than a plain to make the same raise in note. Not necessarily harder to do, but kind of plays with your muscle memory that tends to know how far to bend any given string to get it where you want on any given fret.
The thin wound G is usually a mellow-ish string, where a big thick plain G is brash and loud. Personally, I like the in your face metallic sound of a plain G and that's what I end up with. (Using 10-52 guage nickel strings w/ plain G)
