I swapped my 03 GL1800 for my new 05 K1200LT and haven't regretted it.
Here are a few things that made me switch;
1) I can put a full face helmet in the side bags on the LT and 2 in the top trunk. Can't put them in the side bags on the Wing.
2) Standard ABS on the LT, hard to find on the Wing, or without upgrading to a package with unnecessary options.
3) Standard Heated Grips
4) Much more willing to lean in on curves and better lean angles on the LT.
5) Stronger Servo Assisted Brakes
6) Better brake holding on steep hills, because the hand brake works both wheels on the LT. Its safer to back an LT down a steep driveway especially two up with the front handbrake lever alone. I would drag the front and have no assistance from the rear brake on the Wing in this situation.
7) Smoother more accurate Cruise Control.
8) Much more stable at higher speeds and in crosswinds, no 40-45 mph front wheel wobble.
9) Much more compliant ride than the Wing, the LT is softer
10) Better gas mileage 45-52 MPG, mid 36-42 on the Wing.
What I miss about the Wing;
1) Low end grunt and incredibly smooth engine.
2) Lower center of gravity in low speed manuvers.
3) Better cold weather wind protection
4) The stock headlights on the Wing are amazing, had to add xenon and motolights to the LT to compete.
5) Much easier to "farkle" and remove body work
6) Standard remote locking of saddlebags.
7) Much stronger reverse on the Wing, the LT does not pull as hard and thermally times out much sooner.
8) Better lighting in the rear, easier to be seen.
9) Standard Intercom
I've had issues with both, frame crack recall, overheating in traffic due to air pockets in cooling system, stupid cooling fan programming and reversed fan blowing direction on the Wing.
On the LT, odd radio faults and final drive pinion seal leak, odd spider cracks in the taillight assembly, need to run a battery tender when parked for more than a week.
These heavyweights come with their inherent issues, but I feel the LT is a much more responsive motorcycle once the RPMs are spooled up, phenomenal brakes, larger trunks, adjustable on-the-fly wind protection, more confident in the rain and ABS.
The other reason is dealer customer service. I had to take my Wing to Atlanta, Lawrenceville Honda, 180 miles away for major stuff, local dealer where I bought mine (Icon) wasn't interested in performing warranty work. Bogart's Motorsports has been fantastic with my LT here in B'ham.
The LT to me was a step up, don't get me wrong, I liked my Wing also. The LT is a better "fit". I'm 5-10, 175lbs and 32" inseam, 32-33 sleeve and the LT does sit higher, but I have a better riding position on the LT over the Wing.