Apparently, they switched from spitting slime to shooting needles (or, they were doing it before too, but the graphics team made it look like acid anyway). You can't see the missiles when they attack the battlecruiser, though - I like old attack better.

I like the old attack better myself, I hope they make it so you can see them attacking without having to be 2 inches away from the screen...

They would clearly be visible if it's full screen.

Hydralisks launch needles with the help of their glands. It looks like acid, but I've always assumed that the needles were propelled from the muscules, and that the green stuff was bio-chemicals.

And to the hydralisk burrow, yes it's undrealistic. Looks like blizzard quickly stole the Crypt Fiend's ability and spliced it into here. Sigh.

Yeah it's not like they had burrow in original Starcraft or anything.