Its the vastness of the forms that makes the issue a hard one compared to Klingon ports of OpenOffice.
1) Country Level Forms (US, Canada, UK, etc).
2) State Forms
3) County/City Forms
While the forms do not change a lot, there are constant changes in what new forms and parts must be counted or may not be counted per area. And then you get into cross filing. A person works for you in a state. You may need to collect sales tax from every sale in that state because you have a point of presense in that state (while in another state you might not).
Or the fact that if you provide a cell-phone to an employee that is considered a taxable benefit which requires you to add that to certain forms and not others.