i compared it directly to the previous game from naughty dog that i'd played, jak ii, quite a lot. that's because there were many comparisons to be drawn in u2's favor. after playing the last of us, i view the game as something of a stepping stone. there were lessons they were learning with each game, and the big one from jak ii was to pick a game where a cinematic narrative made sense. they did that with this. the big lesson from u2 was to connect the story and the actions of the player, which they accomplished to great effect in the last of us.
I feel pretty similar as well. I enjoyed Uncharted 2 but thought the gameplay was lacking in some key various ways. I haven't had a chance to play LoU yet but from how the game looked and how ND talked about the game it seemed like it was going to fix a lot of gameplay problems Uncharted 2 had, or as you say, it was a stepping stone.
I really like everything about Uncharted except the part where you play it, I hope Naughty Dog does do an Uncharted 4 on the PS4 taking everything they've learned from their games this gen and making a game that has more interesting mechanics without losing the identity of Uncharted.