Bitcoin is still considered in beta by the core developers.
Basically, there's a blocksize limit in the current protocol. The maximum block size is 1MB. This means only 1MB of transactions can be processed every 10 minutes. During times of peak load, there will be some transactions that are not included in the immediate block and will have to wait. The transactions that pay very low or nothing for transaction fees will be the ones that have to wait.
It is no trivial task to increase this limit as you would need to get the majority of miners to agree with it. Be interesting to see what happens.
“I’m fully diversified. I’ve got some under the mattress, some under the floor boards, some in the backyard.”