In terms of book mechanics EMMachine had it right. In terms of game mechanics...
If you play ironjawz specifically they play a lot like their old solo battletome. Some of the rules differ a bit but the general style is "Hit fast, Hit hard, Hit often". Powerful damage dealers that while initially slow looking can move faster than you think. Problem is low numbers and their defense looks decent on paper, but other than ard boyz doesn't work that well practically.
Big Waagh basically combines the ironjawz and bonesplitterz battletraits all into one, that activate over time as you accrue waagh points(so a resource mechanic) via various means. This means Big Waagh takes longer to spin up and requires you in the end to take some bonesplitterz units(most think the bonesplitterz are lacking) but if you get big waagh fully spun up it is the more powerful of allegiance abilities because you get goodies like +1 to hit AND wound, and a 6++ save to make your brutes and goregruntaz and megaboss that more survivable and a couple of 1 offs to help with magic and such.