What can go wrong when users/miners on the network use outdated versions of the protocol?

As Bitcoin grows in popularity, more developers want to find ways to contribute to the source. Wallets, online markets, etc. have lots of room for improvement and the risks of making changes there seem manageable.

But what about non-trivial changes to the Bitcoin core protocol? There’s always lots of discussion about ways to mitigate risks of 51% attacks and other vulnerabilities. Like adding conditions to accepting longer blockchains, just as an example.

Suppose a big change was actually made, there’s nothing forcing miners to take the update. What are some of the risks of having miners on the network using different versions of the protocol? Not necessarily malicious clients, just outdated ones?

If a big change were introduced, what would things look like as adoption ramped up to >50% of users?