In reply to humptydumpty:
I haven't of course read all your responses so someone may have already listed the lists I feel are essential:
It should have an alphabetically arranged list of crags/areas/sectors (whichever is appropriate). Many guides, including Rockfax Costa Blanca which I'm just looking at, have such a list but it's arranged in page order, as a "contents" list. If I have no prior knowledge of the region or it's geography, and want to look up a place that has been recommended, there is no way of going straight to it.
On the topic of such lists, a route index, similarly alphabetically arranged, is essential, but please please please don't list climbs beginning with "The" under T.
A list of first ascensionists, cross referenced in the text with the date or at least the year, is important. (If you see this Steve Broadbent such cross referencing is missing from your latest excellent Anti Atlas guides).
I've always liked the (necessarily subjective) graded lists. Good fun and at least as valid as the FRCC's recent adoption of e.g. MS-; MS; MS+; S-; S; S+; MVS-; MVS; MVS+; VS-; VS . . . . ad nauseum.
A last point of personal preference which may be controversial as I know many people like the action photos, but I feel modern guide books have tended to overdo it with consequent increase in cost and, more practically, size and weight.