In SlashDot, the opening statement cannot be changed. In Wiki, the discussion can lead to the original statement being refined and improved. Thus Wiki conversations can be more constructive than SlashDot ones. See BarnRaising.
SlashDot provides tiers of influence instead of egality. Wiki is almost a meritocracy except the site administrators have more power (obviously). But all in all, it's all for one and one for all. That makes it fun.