Just curious as I have never owned a Guzzi - how do you describe 'quirky' as I have seen that used several times by owners.
Like the old Brit bikes - Zener diode, Lucas electrics type quirky?
I'm far from an expert, but I'll try to answer this. First there is the styling with those two big cylinders - errr handwarmers - hanging out in front of your knees. The handling on the older bikes was not great - it tends more towards straight line stability as contrasted to the ST's willingness to carve through turns. There is the bikes reaction to revving, it has that heavy longitudinal crank. Mechanically, Guzzi's have had dry clutches, and the Convert had a hydraulic torque converter that made it an early bike with an automatic transmission. Back in the 80's, Dr. John Wittner, successfully applied standard Detroit techniques to increase the h.p. of Guzzis and raced them. Electrically, they are similar to old BMW's and under powered compared to newer touring bikes. I remember back in the '70's Moto Guzzi was considered the ne plus ultra touring bike because they would easily go 200,000 miles, were reliable and easy to service. This was before Honda and the Japanese redefined the market.
I last rode a new Moto Guzzi about 6 years ago at a dealer's demo at a NY rally. I was still on my ST at the time, and I was underwhelmed by the handling, acceleration, and overall feel of the bike. The handlebar controls were not in the 'standard' positions and needed a learning curve to get used to them. Some were downright akward to use. If I were to characterize my 1300 as a modern bike with very good handling, I'd call the Guzzis I've been on as old technology trucks. That's only my personal take, based on a very limited exposure (i.e. riding the bikes) the the brand. My SP1000 was, I thought, a very uncomfortable ride for any distance (it was an '84).
I call them left over paint jobs. They've been doing that for the last 25 years.