I know this isn't the most spiritually uplifting list, but nonetheless, I think they're all things that would make a big difference to the area.
1. A Sainsbury's Local (or even, a Tesco Metro). Some people will be horrified by the idea, and say that it will reduce Brockley's unique character or threaten local shopkeepers, but I'd swap Costcutter for a proper supermarket chain, with fresh food and a big range of products any day of the week. I can't think of any small local retailers who'd be threatened either. Forest Hill seems like a good model for Brockley - the Sainsbury's is nicely integrated in to the high street and there are a range of small shops, bars and cafes scattered around it.
2. A cash machine that doesn't charge. Brockley isn't a northern sink estate, it's a residential area in Zone 2. We're not supposed to be the financially disenfranchised! A proper cash dispenser might even stimulate the local economy a bit - if you have to trudge up to Crofton Park to get cash out, it's hardly conducive to spending locally.
3. A nice, proper pub. The Whetherspoons fails on both counts and is a magnet for swaying, toothless alcoholics. Nice from a distance, just don't look too closely. The Wickham Arms has potential, but is about as warm and welcoming as the Slaughtered Lamb. Except on Thursdays, when there's live music. Actually, a good pub should be number one...