There are many good forum systems out there. Invision, phpBB, XenForo (my fav), and many more.
regardless of their choice, a few features should be there, such as attaching files, images, private messaging, quotes +++!
XenForo juat launched a new version with an insanely good multi quote system which allows you to multi quote througout any post In any part of the forum. Really handy.