The following is based on my experience working on maybe 8-10 debt limit increase laws from both ends of Pennsylvania Avenue.
Both a debt limit increase and a continuing resolution are typically considered must-pass bills. If a must-pass bill is not enacted into law, something very bad happens, such as a government shutdown or the U.S. government defaulting on a debt payment.
Must-pass bills are incredibly tempting for Members of Congress (and outside advocates). Because most bills never become law, everyone wants to attach their desired policy change to a must-pass bill.
Bills to increase the debt limit are always terrible for Congressional leaders and the White House, because they have to get done but no Member wants to vote for them. Traditionally, the majority party (in each body) delivers most of the aye votes and the members of the minority free ride and vote no. That could betricky this year with a Republican House and a Democratic Senate.
Congressional Republicans are excited to use a needed debt limit increase bill as leverage to get additional spending cuts or budget process reforms, as they did with the FY11 appropriations bill.
I can think of at least three reasons why the debt limit fight should differ from the recently concluded battle on FY11 appropriations.