The right mindset to understand the requirements of Bitcoin client design is, I think, that of someone trying to clone an online game client (e.g. Starcraft) in a way that doesn't experience network state desynchronization. You can do whatever you like in your client UI-wise, but the "rules engine" of your implementation absolutely must reduce an incoming event stream into the same world state as the reference implementation.

For this kind of problem, the easiest solution (when possible) is to extract the "rules engine" component of the reference implementation into a library, and then have every other client just use it (and maybe contribute to its development) instead of trying to write something bug-for-bug compatible with it.

When you think about it, web browser rendering has much the same implementation-consensus problems as Bitcoin. And what do we see there? Everyone sort of converging toward just using the "rules engine" known as WebKit.