SP mirror = Continuous revenge killing. Not only did you have to tech for decks, but you had to tech for techs in decks. For instance:
Opponent's Luxray KO's something.
Revenge KO with Promocroak
Revenge KO with Uxie Lv. X
Revenge KO with Uxie Lv. X
And then if you didn't have anything to revenge KO the uxie with you could be in serious trouble.
Although I must admit comebacks still weren't all that possible, especially since Luxchomp could easily snipe a pixie off your bench for the last couple prizes. But think in comparison to this format, now with Catcher. Now it's not just one deck that can ensure a victory off of a prize lead, but any deck running Catcher. So basically any non-trainer locking deck.
Now, I didn't play during the original ex format, but I have heard great things about it. I know everyone on this site has heard someone complaining about the basic power creep, but it really is kind of true.
This isn't to say that the only good decks out there are based around basics, but a fair number of them are. Reshiram/Zekrom variants take up a huge amount of the metagame, because of their speed, high HP, and because they are basic. Just about the only really good, Tier 1/2 decks built around Stage 2's have either Magnezone or Vileplume in them (I guess one could include Chandelure, but it's almost always played with Vileplume anyway.)
With EX's coming in less than a month, and in time for states, we can expect an even larger amount of basic-centric decks, especially the big one, Mewtwo EX. Players are expecting it to be played so much, it's expected to become a staple, because it's the best Mewtwo EX counter, OHKOing it for 1-2 Energy (with a DCE). Plus the Dragons will definitely have their own decks mixed in with the non-EX dragons, and those will be Tier 1.5 at worst.
TL;DR: ND=EX's=Basic dominant format.