For Oregon residents, a PDF of the vehicle code is available at
Oregon DMV 2007 Vehicle Code Book
Search for "Farm Tractor" and you'll find many exceptions to the requirements for other vehicles. There are sections under lights, insurance, registration, etc.
Since I'm not a lawyer I can't advise anyone but the information is interesting.
First thing it does is differentiate between a tractor for farm use and otherwise. I believe that the farmers have had (deservedly) friends in the legislature.
801.265 "Farm tractor." Farm tractor
means a motor vehicle designed and used
primarily in agricultural operations for
drawing or operating other farm machines,
equipment and implements of husbandry.
805.090 Nonfarm tractors. (1) Except
where specific provisions are made for tractors
described in this section, such tractors
are subject to the same provisions of the vehicle
code relating to registration, titling,
transfer, sale and dealer regulation as a motor
truck.
(2) This section applies to vehicles that
would be farm tractors if used primarily in
agricultural operations and that are not
within:
(a) The exemptions from registration for
farm tractors under ORS 803.305; or
(b) The classification of fixed load vehicle.