Ah the advantages of using a spreadsheet for a database. You ruled out the obvious like spaces, characters, quotes, and nulls. Have you rules out the field names like longitude being shortened to long a reserved word? Field names with spaces also cause problems, if you ever plan to export as a shapefile the names have to be 10 characters or less without spaces or risk truncation. Check to make sure the cells also have values not formulas.
Next step if you have checked everything ans it still doesn't work, try to export excel as a CSV and bring that in. If the X,Y fields still don't come in as numbers, you have a character in there somewhere, maybe a comma? Or if you have an older version of excel export as a dbf. Another option use Access, as an mdb files, import the excel table, ArcMap should now be able to read it.
One thing you can count on is excel always being a problem, and people using excel as a database.