Blockchain pollution.
This is different from the Ethereans, who believe that most use cases are fine, as long as people pay the requisite transaction fees.
The initial sync from block 0 can take a while, but after having an almost synced copy, getting it up to date is fast
I remember thinking "this will never work" after reading the bitcoin whitepaper, i assumed that at some point only supernodes would exist. We might be getting there, be we are not there yet, and definitely moving at a slower pace than my initial prediction. Holding 400-500 GB is still very attainable today at a small cost
It led to the transaction pricing mechanism. Every transaction includes a bid on how much you'll pay to have it included in the chain. Miners pick the highest bids from recently broadcast transactions. There's no requirement to do so but it's economically incentivized, as the miner receives the transaction fee. Any sustained attack becomes very expensive. And other users can still pay the asking price to get their transaction in the next few blocks.
Bitcoin has in practice maxed out the great majority of blocks (about 1 MB per 10 minutes) for a while now, and so many cheap transactions just never get processed as they're always outbid.
I took all the public namecoin addresses I could find and put them in a text file and sent everyone dust. Fun times.
What jerks.
Yes, they paid the transaction fee to get put into a block, but now their data sits in the blockchain and eats up storage space on every device that maintains a copy of the chain. For a financial transaction you can at least comfort yourself with the knowledge that the "pollution" is an entry in a finance ledger, but not so here. The transaction fee is set based on how much people are willing to pay right now to get a transaction through, it doesn't factor in the long-term cost to store that transaction on every machine that needs to host it in the future... that cost is ignored.
Seriously, who are you to tell anyone what data is "pollution" and what data isn't? Like, do you go around to people on their laptops in cafes and tell them to delete their files off the Internet because you think they're wastes of space?
EDIT: Having a bespoke data field (OP_RETURN) in a Bitcoin transaction is an improvement on this front, because it provides a cheaper alternative to people storing the same data encoded as potentially-spendable UTXOs.
Of the full chain. You can run a Bitcoin full node in pruned mode, only using the most recent 5GB, with more or less the same security. (A 3-day chain reorg would be needed to reduce the security of your full node.)
It does make it take longer to set up a pruned node, though.
I'm thinking this is more relevant to the vision of ETH being a computer. I can't imagine every memory ever processed being stored into perpetuity.