Firstly, hello again. I've spent the last two months playing Skyrim and getting fat (these two facts are not unrelated).
Guetta didn't just pop into superstar producer status from nowhere - his 'Fuck me I'm famous' nights were a big deal in Paris and Ibiza for ages (playing, oddly enough, American house from the likes of Blaze, Morillo, etc). The likes of Diddy have always had a fascination with European chic and the more glamourous club scene (witness Dirty Money's "Last Train To Paris") and started hanging out at these places on European tours and I believe the collaborations stem from there?
I'd agree with Mistersloane to a large extent - the track he did with Snoop ("Sweat") steals a massive chunk of "Don't You Want Me" by Felix, but if you've never heard the Felix track (either you're too young or too American) that riff is still undeniably exciting.
It's the recognition factor, he always works with a vocalist, and usually a prominent US vocalist who made their name doing r&b/rap. Sia is the only collaborator who could be said to be an unknown, but even then she'd been featured on a track by Flo-Rida (also a Guetta collaborator) prior to 'Titanium' (which is horribly shrill but that's not important right now). Would a Guetta solo instrumental shift units in the same way that Daft Punk or Deadmau5 do? Nah. That's why I'd refute the Moroder comparison - Guetta lives or dies by his guest vocalist, Moroder solo was a viable proposition. I'd compare him more to someone like Ross Robinson, who was the go-to nu-metal producer - he defines a vogueish sound but will be forgotten ere long.
The appeal, then? Familiar singers, insistent club beat, that's all many punters are looking for. Witness 'Scream' by Usher massively outselling the far more interesting 'Climax'.
As has been said, 'Like A G6' trumps Guetta. (And most other things. I recommend the Cataracs highly, they've a new EP out 'Gordo Taqueria' which is excellent.)