It was when I was working on Chambers Universal Learners’ Dictionary in the late ’70s that I suddenly focused on the weirdness of the expression ‘what is it like?’ Why ask for a comparison when you want a description? I managed to squeeze it into the dictionary at W (for what), since it had missed the boat at L (for like). Desk dictionaries seemed not to bother with it. But the 1933 OED pinpointed its function with notable precision: ‘The question what is he (or it) like? means ‘What sort of man is he?’, ‘What sort of thing is it?’, the expected answer being a description, and not at all the mention of a resembling person or thing.’ However, it gave only two citations, the earlier dated 1878, whereas citations from my databases, when I began searching on ‘what … like’, showed that it was in use in the early 19th century. Earlier than that there was evidence that the question was indeed used literally to ask for a comparison.
I’m very aware that ‘what is it like?’ should be studied in conjunction with ‘like that’ (as in ‘He’s like that’, ‘It’s like that’), which can be understood as its counterpart in statement form. Citations for ‘people/things like that’ can be found as early as the 17th century, but the use of ‘like that’ as a complement after a linking verb seems to arrive in the mid 19th century. Trollope, who quibbles over ‘What is he like?’ seems OK with ‘like that’. In The Small House at Allington he puts it into the mouth of Johnny Eames:
‘My belief is, that a girl thinks nothing of a man till she has refused him half-a-dozen times.’
‘I don’t think Lily is at all like that.’