I don't think there was any one thing that switched me onto him, granger. It's a little bit the way travelled so easily, and a little bit how effortlessly he put daylight between himself and his opponents, once asked to go and win his races.
I'd also say that he demonstrated an unusually-high level of both physical and mental maturity for his age, even when running as a 3yo in the first-half of the season. He just appeared to know exactly what his job was. Talent and a straightforward nature is an attractive combo - especially in a juvenile - and he looked like a horse that had been in training for five years.
I had him down as one to follow after his run at the Open meeting, but it was the Finale where I really started to get confident about him. Defi delivered a Jair Du Cochet-esque performance that day, and it marked him out as special (as I said previously; the errors up the straight were borne of boredom, and in no way detracted from his performance).
Like you, I tend to treat 4yos with a high-degree of scepticism, and I surprised myself a little, with how much I was sold on Defi from an early stage. I'm just grateful I called it right.
It should be noted that jinny (I think I'm right here) was the first to mention DDS on the forum. I didn't really do anything clever - as far as I'm concerned, his level was there for all to see from fairly early on in the season, and the only surprising thing about the last several weeks, is that so many weren't prepared to believe it. Any way you look at it, there were no holes at all in his form prior to the Festival, and I found it bizarre that he drifted to as big as 11/4 on the morning of the race.