I think the biggest explosion in the Nunes memo is the one that the memo seems to want to downplay.
The memo notes that the counter-intelligence investigation began in July of 2016 based on George Papadopoulos's drunken encounter with the Australian ambassador at a bar in London. The FISA application for Carter Page came in October of 2016 and was based, in part, on that Papadopoulos episode.
Additionally, information from the Steele dossier played a role in the Page FISA application.
But this is the explosion, though more a bang than a mushroom cloud: the memo downplays the Papadopoulos bit, putting it at the end of the memo, but chronologically it came before the FISA application and led to the FISA application when coupled with the Steele dossier. The investigation had already begun because of Papadopoulos, not because of Carter Page. Page got dragged into it. There is no evidence of conspiracy between Page and Papadopoulos, but as the memo notes, "The Page FISA application also mentions information regarding fellow Trump campaign advisor George Papadopoulos, but there is no evidence of any cooperation or conspiracy between Page and Papadopoulos."
How is all this an explosion?
(a) This was a counter-intelligence investigation, not a criminal investigation like some have claimed.
(b) Papadopoulos's incident triggered it, not the Christopher Steele dossier.
(c) Nunes, by pushing the precipitating factor to the last paragraph and focusing on Page, downplays that.
(d) The whole thing again reflects the lack of any tangible connection directly to the President. Papadopolous was a bit player in London and Page was and is an idiot.
(e) I'll just quote from the memo, "Deputy Director McCabe testified before the Committee in December 2017 that no surveillance warrant would have been sought from the FISC without Steele dossier information."