You will be able to place lava.

The only griefing I can see is that people would still invite others to come to their place and then kill them with the lava. But again you don't really need lava alone to do that, pitfall traps/etc can easily achieve the same end.

I wonder if they'll make it so there's a list of all the deaths perhaps during a single day, and where they were when they died as well. That way people can't lie and say "X killed me with lava!" because, well, the death message would either agree or disagree with it. And then you can tell for sure if someone decided they wanted to get a lot of people killed at their place who exactly died and such.

Just one way to prevent a problem that I know happened at least once in V5 and was a hassle to deal with.