On a server, I'd definitely say the community is important. If there's a good community and moderation team it's an all round better experience - you can play with no drama and no fear of your builds getting griefed.
My build style varies depending on the theme of the server and what would be interesting to build. The most common thing I build are castles and rustic houses. I'd say 75% of my time is spent building.
I've played on many servers over the space of a few years, enough to know which ones I'll have fun on. I've ran a server a few times, too. The last time I did, it wasn't very successful. Here's a link to the server's website (which I've left up for some reason).
I'd say destruction of buildings, placing blocks everywhere and stealing counts as griefing. The line between griefing and a prank is if buildings are heavily damaged and a random person has done it, it's griefing. If it's there is barely any damage - it's funny - and it's someone you know in-game, it's a prank. Pranks are fun, once I farmed eggs and filled someone's home with chickens ;).
A few builds and a lot of drunken ramblings.