You can see this happen when you compare Eligius vs other pools: Each pool can control what transactions get included in a block, usually based on fee's, size, etc. Most operators use a client which prevents non-standard transactions (really really unusual ones) from being included in a block. Given bitcoins scripting possibilities, there are still only two particular transaction types that are standard at the moment. Eligius will mine non-standard transactions into blocks, even though no one else will.
Standard transactions:
Pay to pubkey hash
Pay to script hash (very particular circumstances, ie, m < 3 && n < 4)
- These will not be forced into a block, they will only be included if they have an adequate fee.
Non-standard:
p2sh addresses where m > 3, n > 4.
https://en.bitcoin.it/wiki/Script#Transaction_puzzleother unusual scripts.
- If you broadcast these, the nodes your connected to probably will not even relay the transactions to others. It has very little chance of propagation. It would need to be directly pushed to Eligius in order for it to go through.
Even though the majority don't allow non-standard transactions into the chain, one someone does, they will eventually trickle into blocks (With Eligius at 15% hashrate a non-standard txn with sufficient fee should confirm within an hour), and at which point they can be spent.