There is a well known, much feared (by developers) and often ignored (by management) term for this: Scope Creep.
Seriously, it's in Software Engineering management books from the 1970s.. and it still happens today.
Whilst I sympathize with the lead for wanting to make an awesome product, he really should have stuck to the design doc..(unless there wasn't one in which case the product can NEVER be considered to be finished)
One of the best games in recent years was Portal. It was brilliant because it was simple.
What would Portal have been like if it had Snow scenes, tonnes of scripted sequences and AI? The HL engine supports it all, but they held back and focused on making an awesome game.
Less Is More!