What makes me wonder is if its really a hasty reaction to the american intervention or because of various (my own estimated) reasons:
1) As i said, ISIS strength lies almost solely in their ability to inflict, well, terror. when they show their defiance against americans, they strengthen the notion that they can not be opposed.
2) ISIS ability to recruit members is based on two main reasons: Fame and Agenda. by involving the west, they are drawing the radicals that were on the fence because they saw that ISIS doesn't make too much threats or actions against the West/Israel. now that they are dragging the Americans in, they will not only get more donations, they will get more recruits.
Fame is self explanatory. despite all their horrors, the beheading of the american journalist caught the media attention and brought them back to the front pages. also worth mentioning that their recent showing in the western media was related to their failures against the kurdish rather than to their success.
3) Al qaeda profits greatly from capturing westerners and then releasing them for ransom. at first, they beheaded their kidnapped westerners to show they are serious about their threats (look back at the rate of westerners beheadings by al-qaeda in the past in comparison to the present. they may also capture less, but the relation is not linear. they simply release more for profit). maybe ISIS plan to move to that profitable side of terror.
http://www.nytimes.com/2014/07/30/world/africa/ransoming-citizens-europe-becomes-al-qaedas-patron.htmlSo in essence, just like what Leafsnail said, this beheading is less about americans and more about ISIS.