Sandbox vs Plot I feel can't really be divided as such. Parties can throw curveballs and take wild turns. You can try to provide a framework as a DM, but unless you railroad there's no gaurantee it'll happen. Provide a plot option or ten, and be ready to improv like a mad cunt.
Players relation to the setting again depends entirely on the players and the setting. Extremes are retarded, though, like being a gender-ambiguous vampire were-bunny from the Cthulhu planet in Forgotten Realms.
Gameplay should, ideally, be a really nice sampler platter imo, and players will eventually niche themselves.
I -would- like arcs if I ever found a party that had the attention span to play a single campaign longer than a couple of months.
Boss encounters in RPGs are sort of a staple of RPGs, but if you're going for a grittier/more realistic feel, they can break that effort.
All of those setting options can be pretty fucking rad. Never thought a city campaign would actually be fun or engaging until I ran one, and the players loved it and so did I.