📄️ Privacy
Satoshi left exactly one privacy problem open in the whitepaper, that transactions with multiple inputs "necessarily reveal that their inputs were owned by the same owner." In early bitcoin software, this was true. But no consensus rule prevents transactions with inputs from multiple sources. Payjoin is the simplest solution.
📄️ Scaling & Fees
The Bitcoin blockchain is limited by block size. Approximately every ten minutes a new block is added to the chain. New blocks are limited to around 2MB of data on average.
📄️ Lightning
The Lightning Network (LN) is a second-layer solution built on Bitcoin that takes transactions off-chain to allow for near-instant, final settlements with far lower fees, tremendously increasing transaction throughput, improving privacy, and allowing for new use cases for Bitcoin such as micropayments. It uses a network of payment channels between nodes to route payments from source to destination. These channels require node operators to lock up “liquidity” (bitcoin that can flow between one node and its channel partner) between their channel partners. How much bitcoin you can spend in a channel is limited by how much liquidity exists on your side of that channel.