1. Transactions will get slightly cheaper, but only because they're increasing the block rate from every 13 seconds to every 12 seconds, not anything specific to the other cool stuff in the merge
2. The network will switch from proof-of-work to proof-of-stake, meaning that there will no longer be GPU demand or substantial energy consumption attributable to the Ethereum network.
3. For a cryptocurrency to be secure there has to be some barrier to participating in consensus. Now, instead of having to have a fancy GPU to run an Ethereum validator, the barrier will be that you instead have to "stake" 32 Ethereum (and risk losing it if your validator misbehaves). A staking reward of (I think) 5% a year will be issued for your trouble.
4. The Ethereum network will be more resistant to "short-range" forks, that is, forks that diverged from the proper chain "recently". (More resistant in the sense that it will be more expensive to execute an attack like that.)
5. Once you've staked your 32 Ethereum, you currently can't "unstake" it. The ability to withdraw your stake will be added in a future eth fork. It will have to be gated by some delay (maybe you can only withdraw your stake 6 months after you staked it). Attacking the chain via a "long-range" fork, that is, a fork that diverges from the proper chain longer ago than the withdrawal period, will be much cheaper or possibly even free.
6. Ethereum will still favor liveness over consistency. An attacker can't stop the chain, but they can prevent it from finalizing for a time (at expense to themselves).
7. Token issuance will go down, probably to below the burn rate, so the base Ethereum supply will go down over time
8. The chain will get deterministic finality after some number of blocks (I think something like a day's worth). That means that, once a block is finalized, it will never be rewritten. Network outages or attacks can prevent blocks from finalizing.