As the language suggests, the waiver bars any user from entering a class action lawsuits against Microsoft, requiring instead that all disputes be solved by "informal negotiation." Should said negotiation fail to solve the dispute, individuals may enter into binding arbitration with the company.
Microsoft certainly isn't the first company to add such language to its terms of service, as mentioned above, and it likely won't be the last. The company has dealt with the looming specter of class action lawsuits before, one as recently as 2010. As with other similar terms of service changes, users may choose to reject it by sending a letter (a paper one) to Microsoft Corporation, ATTN: LCA ARBITRATION, One Microsoft Way, Redmond, WA 98052-6399.
We've embedded the relevant sections of the terms of service after the break.
Update: It turns out that you actually can't opt out of the arbitration agreement, at least not if you want to keep using Xbox Live. Microsoft has informed Kotaku that the ability for customers to reject changes only applies to future changes to the arbitration agreement. From now on, individuals wishing to file a dispute may do so by visiting xbox.com/notice, filling out a form and mailing it – again, an actual paper letter – to Microsoft.
Should the dispute not be satisfactorily resolved in 60 days, users can submit an arbitration claim (PDF), an onerous process requiring fees, multiple copies of forms and, not least of all, an attorney.