The technical details on large page allocation: http://blogs.msdn.com/b/psssql/archive/2009/06/05/sql-server-and-large-pages-explained.aspx

The two limitations with large page allocations are that the memory must be contiguous and that the memory allocation is static once allocated (no more memory can be allocated or released). If there are other things allocating memory you can easily end up with multiple chunks of memory that combined exceed SQL's max server memory, but are each much smaller. This is almost expected with later OSs, because they randomise the memory addresses for apps and kernal code to avoid some overflow attacks.

The contiguous requirement can mean that SQL takes quite some time to start and once it has, it's got much less memory than it's allowed to use.

To be honest, large page allocations is not something I would generally recommend.