While most modern saddles are more triangular in shape, the Brooks saddles are rather "pear-shaped" with a wide rear and narrow nose. I find that this shape is more comfortable and produces less chafing on my thighs. But then, I ride a narrow Q crank - with wider cranks this chafing on most modern saddles may be less of a problem.
The narrow nose limits how far forward the rails can be widely spaced (to fit a seatpost) before they narrow toward the nose. You don't want the rails to stick out underneath the saddle!
The narrower the Brooks saddle, the less setback it allows. A Professional will sit about 1" further forward than a B-17 when pushed back all the way. (I found out when I replaced a B-17 with a Pro on my Rivendell - thank God for those relieved clamps on the Campy NR post that Chuck Schmidt described in an earlier post.)
Fortunately, the 74 degree seat angles of a decade ago are out of fashion again. (Never really were in fashion in Italy - my 1988 Reparto Corse Bianchi had a 72 or 73 degree seat angle.)
I find that with a Campy NR seatpost, I can use most Brooks saddles with a rather backwards position. Obviously, you want to stay away from many modern posts that offer no or very little setback.
My feeling is that bicycles are designed to work together. Take a 1990s American racing frame with a 74 degree seat angle and a "modern" 0-setback post and wide-Q cranks, and it may work fine with a modern saddle, but not a Brooks. Change one factor of the equation, and it is likely you have to change others too. Fortunately, there being so many factors to the equation, it usually is possible to make it work.
Finally, the old Simplex seatposts offered more setback even than a Campy NR.
Jan Heine, Seattle