On the side of Destiny 1.5 is a similar UI (which I think is a good thing), retained core gameplay (also a good thing) and same classes (boo!).
Against it are all new destinations, a complete reworking of public spaces, bumping the player count from 12 to 26 in them, reworking game flow (Director in the UI, local maps, etc.), the new and revised enemies (we fought Grunts, Jackals and Elites in nearly every Bungie Halo game, didn't make them any less of a full sequel), new weapon types, reorganized weapon slots and weapons that fit into them, a new campaign (said to be larger than the first, and most assuredly better from what we've seen), reworked Crucible with new maps and game types and focus, lore/Grimoire brought in-game, updated graphics engine and the clan and LFG systems. That's not expansion territory. We've gotten a big expansion with TTK, and this towers over it in terms of overhauling the game and building it out. And huge swaths of the game are still not yet shown or talked about.
I'm open to revising this view if some of the promises don't pan out, or if the changes are more shallow than they appear. But right now I see a sequel that kept what was good about Destiny 1 and reworks everything else, with a disappointing lack of new classes.