Which components actually make a difference?

Sure, but very few of us can afford to have a bike that is high quality throughout (or can't ride it daily for fear of it being nicked). Also lots of people have said that, purely in tems of fun, their preferred ride is actually their rat-bike/beater, so there are also more mysterious factors that give a bike its character. My purpose in starting this thread was basically to find out where people think you should spend your money to maximise immediate riding pleasure (other than beer). FWIW, my fixie goes roughly as follows:

High-end: brake blocks
Mid-range: chain, inner tubes?
Low-end: wheels, brakes, saddle, bar tape, sprocket, tyres, finishing kit
Beater: frame, forks, cranks, chainring

...and I enjoy it a lot.

Immediate riding pleasure around london?

Beater: Everything

Or else you'll worry about it getting nicked, scratches, or otherwise indra'd all the time.

On my SS the parts break down roughly:
Inexplicably good: Fork
Reasonable: Headset
Beater: All the rest
Why is it an oval shape?: Chainring