Sorry if I've been unclear: possibly my views have evolved in response to the well-informed debate on this forum.
Farringdon is likely to be an extremely popular interchange. My main point is that I think there will be a surprising number of people interchanging there who are not familiar with the rail system and may well have mobility problems: far more that is usually seen at a more typical Tube interchange like say Oxford Circus. Farringdon will be more like a London Rail Terminus than a normal tube station in this respect. I just hope that TfL's passenger flow modelling and customer assistance planning has taken this into account.
My reasoning is that there are a large number of origin-destination pairs in SE England where anyone specifing "Minimum Changes" or "Step Free" into Journey Planners will be given an itinerary via Farringdon. Even if it would be quicker to go via the Circle Line, or get a fast train to Kings Cross, or change again at OOC to head West.
(And when you say "nobody is going to be dwelling on the platforms long", I've known someone who has waited nearly an hour on the platform at Ely for the train to Cambridge, because all the trains before then were destination London. Yes, really. This is going to happen at Farringdon as well, especially on the westbound platform where the signage will say something like "Next trains from this platform: Heathrow, Paddington, Heathrow, Maidenhead, Heathrow, Reading, Heathrow, ..." Let's hope the old dears going to Reading have somewhere to sit down.)
Ah I see - well there will always be passengers unsure of what happens when a new railway opens. That being said, lots of passengers experience new journeys all the time and so long as signage and passenger assistance is available and sufficient then this will be no different.
As for "nobody dwelling on platforms" well unfortunately there's only so much you can do! You raise a fair point about people not being aware of fast trains from Paddington and hopping on the next available Crossrail service. In this regard, it depends what journey planners suggest. That being said the Tube runs through destinations in C. London to far flung places, so I don't think its a major issue.