Here's my 10 cents.
I was a fairly devout Halo guy up through 3 and Reach, Reach broke the formula, 4 and especially 5 were failures.
5 is the only Halo I have ever sold off, that should say something when in some cases for friends or LAN reasons I own multiple copies of some. I was also in several clans between H2 and Reach.
Destiny I played hundreds of hours into since day 1. But I never got to Mercury or even got to play the last Raid due to a lack of friends online. The friends I did have either played more than me so the light level difference was too great, or played less than me. LFG or sherpas were not always helpful or available. Destiny 1 did have its share of problems, many being more prevalent prior to each DLC update.
Destiny 2, I think honestly as far as the campaign and public world, fixed I dare say everything that was a concern.
The individual planet maps are much larger, they have many more routes and caves and detail. There's a lot more vertical spaces to explore. If anything is a triumph, they turned the annoying patrol missions that had to be "ground into the fossil record" into missions that actually feel like a story mission. You may end up still collecting 15 of some thing, or planting a beacon, or killing a boss, but the process of getting there is more involved and feels a lot less like boring grunt work of going here, do the thing, rinse repeat ad infinium. The music is more varied and unique to each planet or enemy, it's also much more "in your face" about being themed to an activity.
Weapon play has always been Bungie's strong suit, and it continues. The weapon slot changes I can understand better now is a result of simply having more enemies rush the players. You need to weapon swap rather than reload, and for that they introduced two primaries. Weapons too, seem a little better modeled and segregated. Hand cannons are sufficiently punchy. Snipers are punchy and slow. Pulse rifles and fusion rifles I hate so I won't speak to that. Scout rifles, are somewhat neutered. Swords are back, and there's far more variety of them for different uses. Rocket launchers are more rare. Grenade launchers IMO are useless. For a Arc hunter, your trusty melee blade sees "a lot" of use. Light has been overhauled to "power" and thank the heavens, they changed it so engrams drop based off all your gear's average, rather than currently equipped gear. The "traveling to orbit and back down" grind is gone. Fast travel is present, you won't get a sparrow until the campaign is cleared.
Customization has finally come to Destiny and by an large Bungie seemed to listen. Shaders can be applied to ships, sparrows, and various clothes. Mods that offer various buffs or effects can be added to most gear. There's a lot of hidden little missions and things to do. Armor can be spec'd for recovery or speed.
Story wise, Zavala and Ikora are annoying, or barely believable as far as their arcs go, but fortunately, they have new supporting cast that makes up for them being paper thin. Cayde and Failsafe on Nessus is probably the highlight of the game. The final boss fight is wholly predictable and rather unoriginal, but I don't know that they could have really went too many other directions with it. By the time you get to the end zone of the game, you feel a sense of coming full circle, exploring the new zone is kinda poignant. The setup is made for the DLC, there are several big name drops during various missions. Destiny 3's next enemy is somewhat teased as well post credits.
I rented and beat the campaign on Xbone, took about 24 hours doing all the story missions and campaign missions except for the new ones after the ending. There are a few spots where you're either a veteran and you chug through a few enemy spam sections, or you could go with a team and it would have been easy as pie. Strikes were not available during the rental. I finished around 204 power / light.
I've also played the beta on PS4 and PC, and ofc the game in full on Xbone. The Xbone version does feel like it suffers and there were some noticeable frame drops by being limited to 30 FPS. The PC version is definitely smoother and more fluid. I'd compare it to music and tempo or half steps or measures. At console levels, you're forced to issue a command and watch it play out. PC feels like you have that extra half a second or frame that it continues to register what you want it to do and react. PS4 didn't feel quite as rough as Xbone, but it was still under the PC version.
Ultimately, I think Destiny 2 was what everyone had expected from Destiny 1. It was also what many had expected from Halo 5, but got so wholly disappointed by.