Great strides have been made by the FASB and the IASB to converge the content of IFRS and U.S. GAAP.  The goal is that by the time the SEC allows or mandates the use of IFRS for U.S. publicly traded companies, most or all of the key differences will have been resolved.
Because of these ongoing convergence projects, the extent of the specific differences between IFRS and U.S. GAAP is shrinking.  Yet significant differences do remain.  For example:
IFRS does not permit Last In First Out (LIFO) as an inventory costing method.
IFRS uses a single-step method for impairment write-downs rather than the two-step method used in U.S. GAAP, making write-downs more likely.
IFRS has a different probability threshold and measurement objective for contingencies.
IFRS does not permit curing debt covenant violations after year-end.
IFRS guidance regarding revenue recognition is less extensive than GAAP and contains relatively little industry-specific instruction.
Perhaps the greatest difference between IFRS and U.S. GAAP is that IFRS provides much less overall detail. As an example, IFRS fit into one book, about two inches thick while the three FASB paperbacks of pronouncements plus the paperback version of the FASB Emerging Issues Task Force consensuses measure about nine thick, and that doesn’t include all of the authoritative literature.