With almost 300 hours of Destiny 2 (D2) back on Battle.net (the Blizzard launcher), let me give this perspective/advice for new players, specifically with regards for the F2P offering that is Destiny 2: New Light, with a few more holistic thoughts sprinkled throughout:

This overall game is very good. It doesn't always live up to its promise (the first mission of every campaign is always the most polished thing aside from the raids/dungeons), but when it does, it really shines.

The gunplay is unrivaled. The visual/audio feedback is great, the handling feels really good, and enemies really pop when you take them out. Expect nothing less from the studio that brought us grunt birthday parties.

The game is better in a fireteam. Now that the base game is F2P [though with considerably less content than it used to be/I would like], this should be easier than ever for most activities. That said, a huge majority of my playtime was done solo. It can get tricky at times ("Darkness Zones" in certain missions prevent respawning but allow for revives, so solo players have a distinct disadvantage there).

With that said, dungeons (there are several, but 'Prophecy' is free to all players) and raids (again, several of these, but both 'Vault of Glass' and 'King's Fall' from D1 are free to all players) are best experienced with a team, and the new in-game Fireteam Finder feature is excellent for times when you don't want to deal with Discord LFG. Exotic missions, most of which can now be found on a weekly rotator menu option, come close to dungeons in quality while often being more bite-size but almost as rewarding - who doesn't love a guaranteed exotic.

Of the matchmade multiplayer offerings (the game calls these Ritual playlists), I enjoy Gambit (PvPvE, reminds me of the Black Ops Zombies 4z4 mode done right), jump into the occasional Strike (PvE, matchmade fireteams running campaign-like missions), and have no interest in Crucible (PvP, it's fine I guess, but I've never liked PvP that's dependent in large part on personal progression - especially in the competitive playlist, or other competitive modes like Trials of Osiris [behind the paywall] and Iron Banner[F2P]). The free gameshow-satire objective based mode Dares of Eternity is good mindless fun, while the challenging horde mode Onslaught pushes the combat sandbox of the game to its absolute limit. Seasonal content also introduce some great 3- and 6-person modes, but these require additional purchases and are (mostly) purged from the game with each major expansion release.

Don't be afraid to ignore content - this is absolutely the kind of game where you can set your own goals. Want a specific exotic gun? Look up where it drops and focus on grinding that content. Want a specific roll on a random roll gun? Find the best possible farming (or crafting) method and go to town. Chase down some of the exotic questions that the game clutters up your quest page with, or just goof around doing things that you find fun while absolutely ignoring all signposts.

Some stuff just won't be explained in game. It's not as bad as Warframe (much less need for a wiki) but some things are still obscured. Feel free to look up Xur (exotic weapon vendor who visits on weekends) and other hidden exotics tied to specific activities, lost sectors, patrol locations, or quest chains.

For anything that you can't easily find online - reach out and find a veteran player to guide you. The community for this game tends to be pretty great, and enough of us have spent enough time leaning specific parts of it (e.g. raids) or just mechanics in general ("What are random rolls? Which do I keep?") that we'd be happy to jump onto discord (there are a bunch for PC, just check out the subreddit sidebar linked below) and answer any and all questions you may have.

The fact that a representative chunk of this game is free makes this the perfect time to dip your toes in without emptying out your wallet. Any other questions about this game as a product ("Should I pay for DLC? Is Eververse worth it?") should absolutely be researched elsewhere.

If you're looking for a place to start with the F2P content, finish the intro missions in the Cosmodrome. New and returning players are semi-regularly upgraded to a new light level floor, but means that you can miss out on the "learning the ropes" parts of the game that come with working your way through the New Light quest chain or unlocking different subclasses. [As of Beyond Light, all of the campaign content through Forsaken are not currently accessible. Release order was Red War (launch campaign) > Forsaken > Shadowkeep > Beyond Light > Witch Queen, followed by the upcoming expansions Lightfall > Final Shape.]

Eyes up, guardian.

Footnotes for people yelling at me in the comments:
-All of this is my opinion. Feel free to hold your own and disagree with mine.
-I generally enjoy grinding, looter shooters, FPS's, and weird sci-fi space magic. Halo, Borderlands, Warframe, and CoD Zombies are some of my most-played and favorite games.
-As far as I can tell, anything that is not strictly subjective in the above is factually correct [as of Season of Wish/Into the Light, with updates noted throughout. This game changes quite a bit over time, so make sure to do your own research before spending money on it]. If I've missed something, feel free to point it out in the comments and I'll do my best to update this periodically.
-This review previously made mention of both content vaulting and weapon sunsetting. These practices have both been discontinued as of Season of Plunder, and nearly all of the content affected (aside from the Red War campaign and a few destinations) has been returned to the game in some for. I consider this, along with Bungie's commitment to never do either again, as much of an apology as we're going to get -- and an apology that I personally accept.

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TL:DR: It's good. Not perfect, but really good. And since it's free, you can get a really good sense of if this game is for you without paying for any of it. You can play almost any activity solo pretty easily except for a handful of endgame activities. If you're overwhelmed at first, just play through the Cosmodrome missions while you get your feet under you, or ask a veteran player for help.
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Sites to use (List was compiled at Shadowkeep launch, but most of it should still be relevant/useful)

Beginner's Guide (Current as of Lightfall) - https://www.bungie.net/en/guide/destiny2

The DTG Subreddit - /r/DestinyTheGame on Reddit - https://www.reddit.com/r/DestinyTheGame/

Destiny Item Manager (mange your inventory from your browser even while in-game, essential to avoid unneeded trips back to the tower) - app.destinyitemmanager.com

Destiny 2 Checklist (Lists out everything to do as well as when it resets in-game) - www.d2checklist.com/

Light gg (Database of in-game items, useful for looking up random rolls) - https://www.light.gg/

Braytech org (Lots of overlapping features, but I really like their maps) - braytech.org/

Datto (Destiny Youtuber who posts lots of useful/educational content, including exotic guides) - https://www.youtube.com/channel/UCkmAjbIecTIjTQctolYeuDw

If you've played D2 on another system, check out the CrossSave page (https://www.bungie.net/7/en/crosssave) on the Bungie net official site respectively to get all of your content and progression moved over to Steam.

Story Recap up through Shadowkeep (Spoilers Destiny 1 / D1, and D2, obviously) (My name is Byf on YouTube: The Complete Story of Destiny! From origins to Shadowkeep (Timeline and Lore explained)) https://www.youtube.com/watch?v=9mbDXvDNqTU (Lore is one of the things that this game can be so frustratingly about - all of these other stories are so good but really hidden from the player, either because you missed them or they're not properly told to you in-game)