Interesting question. 1/ Great, enigmatic title. A filmgoer who didn't know Bond would say, hm, that's a catchy title 2/ credits sequence with dizzying balls (bezants?) coupled with tremendous music 3/ the great John Barry music of course a critical ingredient 4/ the opening shot of Bond and the MUSIC 5/ Jamaica -- perfect location to introduce the world of Bond 6/ to perhaps overuse a word, the iconic shot of Ursula from the sea (although perhaps a later film would feature Honey as in the books (naked with spending, jutting, etceteras) 7/ Villain. the good doctor must rank in the top ranks of all the Bond maniacs, and Joseph Wiseman is wonderful with his melodrama, no doubt at insistence from the director .... here, we have the 'voice of doom,' plus treating the 'guests' to a gourmet meal.
In the later films (or books, that I recall) we don't have the fantasy of suddenly coming across a beautiful naked girl in the surf; or Bond coldly shooting a man: this sequence, along with the opening shot in the casino, sets the character of Bond for the rest of history.
Goldfinger of course comes close with 'Who are you?' from Jill Masterson (imo, would've made a nice Miss Galore, if she had the acting chops). Also, we have the 'voice of doom' repeated in the Strange Room. Finally, we have what would become a set piece in 'spy movies' -- the futuristic lair with a nuclear reactor, henchmen (and henchwomen in lab coats). Not to forget that Bond dons the gear of one of the henchmen, and is therefore able to throw a wrench/spanner into Dr No's works. We also see Bond engaging the main villain in a duel to the death. The small budget added to the appeal, because the filmmakers were able to show what they could do without the cushion of a big budget. For the above reasons and more that the filmmakers obviously took into account, Dr No was certainly a very good choice to introduce Bond. If it had been done a few years down the line with a huge budget it would of course be more gorgeous with Jamaica, but other than more expensive sets, etc. Dr No is the 'perfect' first Bond film.