Transactions per block growing too quickly for current 1MB size limit (increased size will hurt smaller pools but most people don't give a shit).
In addition to block size increases, there are a couple improvements coming down the line to help with transaction volume:
1) Micropayment channels will allow sites like Satoshi Dice to compress each customer into a single transaction per block.
2) Off-chain transactions will be much more secure when they start using multi-signature transaction to hold funds.
Hopefully (fingers crossed!) these will be in easy-to-use clients before Bitcoin gets much more popular.