Each hash on itself is as hard as any other. The number of tries you need is proportional to the difficulty you had while hashing.
There is no need for an in advance definition of difficulty.
Work for a block = 2^256/(hash value)
Work for a chain = sum of the works for a block.
To produce a chain with a greater proof of work would require that you carry out the same number of hashes.
With a protocol change, difficulty would only be required to determine what the coin base for the block was.
Coinbase = 50*(target difficulty)/(hash value) --- but capped at 50 per block.
With a variable coinbase, it would reduce the need for mining pools.
Nodes could refuse to accept blocks with hashes greater than a threshold, in order to reduce spam.
I wasn't suggesting throwing away the rule that the longest chain is simply the one with the most blocks. I was suggesting to use the hash value as a tie breaker when a fork happens. This doesn't require a protocol change, just a change to how nodes forward blocks and how miners decide which block to mine.