Checklists have been forgotton in the past & it still happens today. The before take-off checklist in the 777 only has one item - flaps. If you forget the checklist due to distraction, there will not be much to remind you to do it.
As for the take-off config warning, well we all know how many times that has not worked for various reasons! I've done a take-off myself (in a 737) with it inop & only realised after TOC when the selcal chime didn't work. An engineer had pulled the 'aural warning' circuit breaker to do some ground checks & forgotton to push it back in. I missed it during the CB check as it was right next to the fire extingusher on the F/O's side & I hadn't stuck my head far enough over to get a clear line of sight on that row of breakers. Good lesson!!!
When I first started flying Boeings, some 20 years ago, we always selected the flaps during the taxi. However, Boeing started changing that a number of years ago on some of the fleet & have now standardised the procedure on all Boeing types, as far as I am aware. I'm sure that they had good reason to do so. I am aware of a couple of collegues who have started their take-off run with the flaps still up. One aborted & the other selected the flaps & continued the T/O, relying on them to reach the correct position before rotate.
There are a number of aspects of aircraft operation where the operator has the option of differing from the manufacturer's recomendations & this is probably one of them. Whether it is a wise decision or not, only time will tell. I do feel that it is an accident waiting to needlessly happen & I certainly don't believe that it is world's best practice.