More than 50% of the mining is done by Chinese pools. That means that they control the network - who's transactions get in, whose transactions are left out. Since the block reward was reduced to 12.5BTC, they are completing it with transaction fees - the bigger the transaction you're sending - the bigger the fee you need to pay them. Last time I've checked I've seen some transactions with fees of more than $1000 worth of BTC in them. It also depends on the
BTC price.So in a sense, Bitcoin is starting to act like a traditional banking system - owned by Chinese people.
Blockchain size is increasing rapidly and soon to reach 200GB of storage - when the size will get ever larger, especially assuming that people that host the entire chain on their computer do not get rewarded for it and maintaining a full node requires electricity + storage + network uptime, it will soon be costly to run a full node. 200GB and even 1TB of storage is no problem for a central authority to run, and it seems to be going in that direction. How can the network continue to be P2P like that? And especially what happens if that main storage will be taken over by the authorities and be shut down?
Downloading and installing a node is costly and difficult, most of the wallets being opened by people today are online wallets, most likely under the main site [Suspicious link removed], and this site seems to be run by ghosts - there's no actual person there that you can contact in case of problems with your wallet, and the only way to contact them is through a Luxembourg court order (see "Legal" section on their site). Same goes for the bitcoin.org domain - there's no person that can take responsibility for these operations. What happens if tomorrow [Suspicious link removed] gets shut down for legal actions? What will happen with people's wallets and their funds then?
There is no governance, and the only thing that governs the entire network is mathematics. What happens if tomorrow all Chinese pools collaborate together, make a Bitcoin transaction of 1M Bitcoins, and after 5 or 6 blocks (confirmation) they start to re-mine all the blocks again in order to erase that transaction from the chain? Assuming that they outline all the other competitors, I can't think of a scenario where they won't catch up and even win over the honest chain, assuming the difficulty changes only every 2016 blocks. If I was the owner of these pools, what can prevent me from deliberately ceasing all my mining power? wait for the net difficulty to lower, and then do this attack?