My mental map of London public transport is now somewhat redrawn with a kind of holy cross superimposed, centred on Farringdon where Thameslink and Crossrail intersect, so that for getting almost anywhere, my first thought is how close can I get using Thameslink + Crossrail.
This is somewhat influenced by the fact that my local station is on Thameslink - but the two lines now form a neat pair of high capacity, high frequency, level-access, relatively fast E-W and N-S routes that meet pretty much in the middle of London and extend quite some distance out of either side.
You can use the combination to get between nearly any pair of London Rail Terminii.
They directly connect Paddington, Kings Cross, St Pancras, Liverpool Street, London Bridge (effectively picking up Charing Cross and Cannon St in the process) and Blackfriars.
Fenchurch St misses out somewhat unless you walk over to Liverpool St.
Euston is a doable walk over to St Pancras.
Marylebone misses out, as does Waterloo.
Victoria itself misses out but most longer distance Victoria services stop at East Croydon where you can get onto Thameslink.
One effect of this is that it's much easier to get a bicycle across Central London (outside of peak hour) because you can take them on Thameslink and Crossrail. Same for people with large luggage, wheelchairs, etc.