The thing is that Difficulty Classes are much lower now. All your saving throws even as you gain levels average out at about 15 or so, which makes buffing your ability scores unnecessary. In my opinion this makes those items that do increase your stats all the more special because now your super-human traits are that much more remarkable. As a peer that is also currently engaged in a long standing 3.5 campaign (4 years and counting and they've attained level 12) while also running a relatively new 5e campaign (a handful of sessions) I can personally attest to the fact that 5e is much more character-driven and relies less on stats; everything has been designed to make the story more compelling.

As far as the rarity of the increases, they've largely left that up to you. They've sort of put ability increases and feats on the same level where you still have the option at levels 4, 8, 12, 16, and 20 to increase your ability scores (one ability by 2 or two abilities by 1) or you can opt to take a feat instead. Once again, I find that this does a nice job of simplifying the game (SO MANY feats in 3.5) which has the added effect of taking the emphasis away from roll-playing and putting it back onto role-playing.

Quote from Braden >>

The thing is that Difficulty Classes are much lower now. All your saving throws even as you gain levels average out at about 15 or so, which makes buffing your ability scores unnecessary. In my opinion this makes those items that do increase your stats all the more special because now your super-human traits are that much more remarkable. As a peer that is also currently engaged in a long standing 3.5 campaign (4 years and counting and they've attained level 12) while also running a relatively new 5e campaign (a handful of sessions) I can personally attest to the fact that 5e is much more character-driven and relies less on stats; everything has been designed to make the story more compelling.

As far as the rarity of the increases, they've largely left that up to you. They've sort of put ability increases and feats on the same level where you still have the option at levels 4, 8, 12, 16, and 20 to increase your ability scores (one ability by 2 or two abilities by 1) or you can opt to take a feat instead. Once again, I find that this does a nice job of simplifying the game (SO MANY feats in 3.5) which has the added effect of taking the emphasis away from roll-playing and putting it back onto role-playing.