The single biggest problem is date / time formats. Excel encodes dates and times as floating point numbers. The date / time-ness of a cell is encoded in its fo

The single biggest problem is date / time formats. Excel encodes dates and times as floating point numbers. The date / time-ness of a cell is encoded in its format. Some of the formats are hard-coded, some float depending on locale, and some use explicit format-pictures embedded in the styles. But of course there's no guarantee that the user has formatted a cell with a date as a date. Sometimes the user has pasted a US-format date into a copy of Excel running in a UK locale, so dates that fit have day and month swapped, while dates that don't fit show up as strings. This is non-trivial to handle in the general case. I've had to add customer-specific workarounds for this case in particular.