Often a speaker that is especially "forgiving" has a somewhat recessed upper midrange/lower treble region. This is the region where the ear is most sensitive (as indicated by the Fletcher-Munsen curve), so any coloration in this region is especially annoying.
The BBC-spec LS3/5a design was I believe among the first to intentionally incorporate a dip in the 3 kHz ballpark region for psychoacoustic purposes - now commonly known as the "BBC dip". It can result in what some would consider an overly sweet and laid-back sound, but it certainly is forgiving. Silverline (among others) offers several models with a significant "BBC dip" in the response, and they are indeed very forgiving and relaxing.
Maggies 3.6 and below are voiced with a broad, shallow dip in the midrange which serves to psychoacoustially balance out the lack of deep bass, and as a side benefit Maggies are exceptionally forgiving.
Also, in general a speaker with a wide radiation pattern tends to be more forgiving than a narrow-pattern speaker. Examples include models from Shahinian, Dueval, Beveridge, Sound Lab, MBL, old Epicures, and Bose (yup - that and a severe BBC-ish dip are Amar's secret weapons).
