Raising fees. I think that fees are extremely low and should be considered at more realistic level, say 0.05 USD/tx. Also miners would welcome this 1000 times
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The protocol can't enforce fees. The reference rules to prevent DOS attacks impose a required fee on LOW PRIORITY TX only. Miners are free to choose what transactions they include in a block. If you run a mining node you are free to exclude ANY transaction you wan't (for any reason). Raising the mandatory fee on LOW PRIORITY TXs would be foolish and not have the desired effect you think it would as most transaction are HIGH PRIORITY and thus aren't affected by tx fee rules.
Pruning. Spent transactions will be possible to have pruned one day. This would be a relief from Satoshi Dice, however, I can still imagine millions of spam addresses with unspent micro-amounts.
Anti-spam rules make this prohibitively expensive. The attacker would end up paying 10x to 100x the cost incurred by full nodes (and light nodes would incur no cost). It is like trying to outspend your enemy when you enemy is thousands of users and it cost you $100 for every $1 it costs them. Futile and stupid.
I think if not addressed then it will be embarrassing for new users to download some 4 GB of blochchain next year.
Why? There is no need for users today to run a full node. If they wish to run a full node then the 4GB "cost" is something they accept.
I hope I am not wrong here but something about fees may be encoded in the software, like not to transmit low priority transactions with fees under 0.0001 BTC. Anyway, I understand that the fees level to include low priority transactions in blocks is some general agreement among users. Indeed, I'm suggesting to raise this level, because I understand, that both Satoshi Dice and any possible spam fall under low priority tx, and it would help to keep them at more acceptable level.
Yes, at first sight the $10,000 buys a 1 GB of blockchain. But this will be transmitted, stored, re-transmitted, downloaded, etc, eventually forever. I think more of some moral harm to the project or loss of momentum, e.g. if such spammers grow the blockchain to 10 GB this year then many full nodes may just decide to stop. But that's just a matter of opinion. I still think there can be guys who may find the network endangering them, and go out and outspend it.
Indeed, non-technical type of users are not supposed to be running the reference client (something in the sense "Bitcoin-Qt - cool; 2 GB of data - wtf"). Somehow I find this unfortunate for the project, but again it's just a matter of opinion.