1. Identity
Wallets are required to interact with chain. This means every user has one which is great because you can use wallets to sign message and authenticate people.
People's transaction history are public so you can fetch that and do authenticate on a micro level. Say, you want to authenticate people who have a particular nft or spent money somewhere specific.
2. Tokenomics
Encourage contribution and investing by providing financial motivation in the form of tokens through technical means.
Take example of torrent protocol. A big problem with torrent is there are not enough seeders because there is no incentive to seed which means many useful torrents go down.
3. Public ledger
This one doesn't need much explanation. Use cases like DNS or voting fits the area where every record needs to be publicly available and verified.
4. Financial & payments
Prediction markets, Programmable finance, cross bridges, AMMs, lending, yield farming, etc. Some are cool, some are almost scam but there are some novel ideas tested in practice here. AMMs are one successful example.
5. Collectibles and scarcity
This one is controversial. If you know the traditional gacha and pay to play industry is huge, then you understand the applications where trading and uniqueness is built in. The items can be made unique and be used outside the game which increases their value.