Block Size Debate
A ongoing debate that started in 2011 (by devs like Gavin Andresen, Jeff Garzik and Mike Hearn) about how to scale the Bitcoin network.
Some blockchains have fixed sizes, like the Bitcoin Core protocol, that limits blocks to 1 MB in size (9-2019). Each block contains at most some 4,000 transactions. Blocks are added to the blockchain on average every 10 minutes, therefore the transaction rate is limited to some 7 transactions per second (TPS).
The debate raged on for years on r/Bitcoin, but when it got censored heavily by the administrator Theymos, it split off to r/BTC.
Many projects forked off from the BTC chain (the most famous one is Bitcoin Cash, which now is the coin talked about most on r/BTC) and many alt-coins were created to fill the gap.
For a long read about the whole timeline, go here.