I first saw McEnroe play (on TV) when I first started watching tennis in 1989. Obviously he was past his peak, and being that I knew nothing about tennis at all at that time his ability as well as that of the other players passed me by. As the years passed and YouTube came around, I was finally able to fill in some of the gaps of the older players at their best.
I still don't know what to make of McEnroe to be honest. To my eyes he has what I would call a somewhat 'shaky' ground game - he was like Edberg in that he always wanted to come in to the net at the first opportunity, but I never felt he could last long enough with the bigger hitters to get there enough (at least after 1984). Before, sure. Now, whether this was due to his long break, lifestyle choices, improvements in rackets and players like Lendl and Becker - who knows. Probably all of the above?
Sadly, he really is remembered more for his outbursts than anything, and for that he only has himself to blame. In terms of talent - super talented, but basically built for another time/era. It still really haunts me to see footage of him, say at the 1989 Masters (WTF in those days) against Lendl, and witness the absolute hammering he takes both at the baseline and the net. But in his prime, he was of course magnificent.
So - in my opinion not as talented as Federer, but a completely different way of playing that makes it extremely hard to compare. And yes, he didn't use many different types of spin on his groundstrokes, from what I can tell.