...would it kill you to give the slightest acknowledgment to people who open doors for you/let you go first on the bus/step into the gutter as you walk two abreast down the pavement?
No need for a gushing thank you, but a nod, 'cheers', quick smile or even basic eye contact would be alright. Rather than sailing past with your nose in the air.
(Brand-new parents get a pass because you must be bloody shell-shocked but the usual offenders clearly aren't at that stage.)