2001 - 2005 GL1800 or GL1800A (ABS Brakes) No major changes during these years.
2006 Enlarged radiators and fans along with upgraded radio and speakers.
2006 - 2010
Model I - Pemium radio and comfort (seat & grip heaters)
Model II - As above plus Navigation
Model III - As above plus ABS Brakes
Model IV - As above plus Air Bag
2012 - 2013 Improved Suspension, Enlarged Saddle Bags (not by much) Improved navigation on Models II - IV. Models with options correspond to earlier models.
I probably missed something but this will give you the gist of it.
FWIW Jon, the 1800's are larger, heavier, torque-er (made that up!), such that when you go back to a 1200 you feel like you can flick a 1200 around like nothing....IMHO you will also be riding one of the most refined bikes out there (never rode a 1500 though), if both were brand new i'd go with the 1800.....jmho
Some items that I considered while shopping for mine (that I didn't see listed above):
Weight can be "discussed" - the 2005 lists as 792lbs in many places, the '06 Premium Audio (base model) as 822lbs (838lbs as a level III), the 2012 level III wet weight (oil & fuel ~50lbs) as 908lbs, so 859lbs dry... My '88 1500 was labeled at 793+lbs; documentation for the latter year SEs show nearly 820lbs... (feel free discuss amoungst yourselves!)
2004 - Handlebar/Radio controls were illuminated & a heatshield for the rear caliper was added
2005 - Frame was strengthened (putting to rest most concerns over cracking)
2006 - aside from the cooling upgrades (above), changes were made to fairing to accommodate the radiator sizes, dash-vents dropped, Saddlebag lighting was changed - also more of the smaller-style environmentally sealed "Amp" electrical connectors are used to replace the older Hitachi open-style used since the beginning of the GLs
2008? - I'm not certain where, but the base weight seems to have grabbed about 7-8 lbs and a bit more again in 2012
2011 -- worst GL1800 avoid at your peril :cheeky1:
2012 - body/suspension redesign as mentioned
"Fully Loaded" would probably be something like Airbag, ABS, XM radio, Navigation, and the heated grip/seat "Comfort" package ?? right?
satan, the suspension redesign mentioned on 2012s is bunk written by some nimrod magazine writer. I just last week upgraded one to progressives front and rear, the springs we took out were just like the earlier models, owner reports a very noticeable difference.