So here's the book where the James Bond formula really takes shape IMO. I think it's a vast improvement over Casino Royale, which wasn't a bad book by any stretch of the imagination.
First and foremost, Bond now has a sharp wit, like in this exchange:
Captain Dexter: This case isn't ripe yet. Until it is, our policy with Mr. Big is "live and let live."
Bond: In my job, when I come up against a man like this one, I have another motto. It's "live and let die."
There's also a real comraderie between Bond and his allies, especially Felix Leiter (which makes his eventual fate all the more horrid). I suppose today their relationship would be deemed a 'bromance.' So where CR's Bond is pretty much all business, LALD's Bond knows how to enjoy the company of friends. I especially enjoyed the way Bond and Felix gape with horror at the retirement community in St. Petersburg.
Bond's romance with Solitaire is more flirtatious and less melancholy than with Vesper. Not as deep, but fun. I wouldn't say that one is better than the other, but the disposable Bond girl template supports more adventures. They can't all be Vesper Lynds, after all.
Mr. Big is a larger than life supervillain, a meglomaniac who brings you into his lair to reveal his evil plan. I found him enjoyable if a bit underutilized. His plan to dispose of Bond and Solitaire by feeding them to sharks is over the top fun, as is the voodoo motif.
Fleming successfully grounds even the more fantastical elements in real tension, like when Bond contemplates drowning Solitaire and then himself to spare them being eaten by sharks and barracudas.
Fleming also seems to strike a better balance between action and luck. Bond escapes Mr. Big's clutches in Harlem by punching and shooting his way through some henchmen before taking off in a stolen car. And he sets the mine that blows Mr. Big and his boat to bits, although if the timing had been off by a hair Bond and Solitaire would already be dead. The message is clear: being good at what you do isn't always enough, but it sure doesn't hurt.
Also of note: Fleming lends authenticity to Bond's travels (he's clearly familiar with Harlem, Florida, and Jamaica) and to his tastes. Bond enjoys jazz, but hates the way Americans cook their eggs and design their cars).
I think Fleming's real accomplishment here is making Bond versatile enough that he fits comfortably in 'grounded' adventures and more exotic ones.