Having re-read this, I do regret my polarizing tone, which I didn't intend that way.Re-doing your example:
1) Fiction first: Let's design a new spell that is fun and cool for wizards! OK, what should it be? How about a lightning spell? OK, cool, should that be like Dark Jedi lightning, or a lightning blast? And what happens when it hits water or a wall? Are we trying to simulate real-life or just a basic suspension of disbelief? Hey, are there any other lightning spells already, because it would be inconsistent if magical lightning spread through water in one spell but not with another. Then, what spell level is this? Does it work with game balance?
2) Mechanics first: Let's design a new Controller class! Cool, so obviously, we need a damage + push spell. OK, um, how about Lightning Ram -- a blast of lightning pushes you 3 squares. So, um, do we have to worry about the lightning ram reflecting off walls and water? Do we have to worry about consistency with other lightning spells? Silly, of course not! We decided we need a Controller because the supplement needs more controllers. Controllers need a damage + push spell for this level slot. The fluff is irrelevant, let the DM and players figure it out. We'll tell them something like 'The game is yours, YOU decide if it's lightning ram or football ram or whatever, use your imagination!'
Fiction first --> what can we add to make the narrative more fun and fantastic, then see how it fits the rules and game balance.
Mechanics first --> what can we add to game mechanics and tactics, then let the people reverse-engineer the fluff and narrative.
I do criticize 4E for being so outwardly gamist IMO, but earlier editions of D&D are far from perfect.
It's just that on a scale of 100% fiction first (=fantasy novel) to 100% mechanics first (=abstract board game), I feel 4E is a matter of degrees higher towards nakedly obvious mechanics first.