It's actually not really optimised for micro transactions.
The popular gambling site satoshidice is responsible for the rapidly increasing size of the blockchain, now around 3GB.
Bitcoin has two main utilities, firstly it is very hard to prevent a bitcoin transaction from taking place, it can potentially be tracked after the fact if you aren't careful though.
Secondly the fixed limit means that it should become a safe store of value, assuming that bitcoin is successful in the longterm.
Bitcoin isn't really designed to scale up to VISA levels of transaction processing, instead it is more suited to be used as a clearing house, to track deposits and withdrawals.
Bitcoin can scale to VISA level transactions[1]. The software is still BETA, the optimizations required for this scalability have not yet been implemented (they are being actively worked on). The primary one is support for pruning the block-chain. This will drastically reduce the current 3GB blockchain size.
You are right though, in it's basic form, Bitcoin will not work for micro-transactions where you want to transfer a few cents a few times a second. However, there are strategies [2] to support some forms of micro-payments.
The pruning stuff looks great, at the moment the time it takes to download the blockchain (and the disk space requirements to a lesser degree) is creating a disincentive to run a full verifying node.
I suspect bitcoin blockchain transactions will never be very useful for micro payments, because of the confirmation delay.
If you use bitcoin as a clearing house though, which means you aren't broadcasting a blockchain transaction for every payment then micro transactions could work very well.
It is still optimised for micro-transactions where that 4% fee isn't a significant cost.