As someone who used to re-roll characters all the time:
You'll lose any unfinished quest progress, all sub-class progress, and all equipment you don't remember to put in the Vault first - basically, apart from stored gear, it'll be the equivalent of never having had a Titan and rolling a new character from scratch.
Bonuses: You'll get to replay the campaign (if you didn't know this, they update some of the dialogue every release, so it's worth listening to Ghost and watching the cutscenes if you're into that sort of thing), you can re-earn some of the Exotics or pick a different free piece of armor if you haven't gotten it yet, some existing gear can be equipped early enough to use during parts of the Campaign (depending on LL - I think you can dip into Collections as well if you really want certain Exotics early and have the necessary Shards), and once you hit 50 you can pull all your Vaulted gear and skip the grind to increase LL.
Basically, getting from 1-50 will be the same as the first time, and 50+ will let you jump right back in, but you'll have to re-meet any prerequisites for activity access and restart any Quest lines.
English
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So I’ll have to reacquire both my seeds of light?
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I believe so, yes - I haven't tested this in Y2, as I'm still leveling 2 characters and haven't even gotten my Warlock past Level 31, but that's how it's always worked and I'm almost certain Seeds of Light will work the same as normal sub-classes (i.e. when you start a new character all progress is lost and you have to re-acquire the subclasses from the Shard and re-level them, just like D1 started with the default Subclass and the TTK subclass had to be unlocked again - although, on subsequent playthroughs, they DID unlock the second subclass as long as they player had reached Level 15 on a previous run, but that was removed in D2 due to the Shard quests). Technically there's no such thing as "re-rolling" a character, only starting a new one - D2 has never had a mechanic to differentiate a "second" Titan from a "first" Titan, only that you're playing on a Titan and haven't currently met the requirements to unlock those abilities. (Same applies to Quests, or the Campaign, or any other element of the game - if you start over, the game treats it like you're starting for the first time and ignores previous iterations.)