Starfield's Universe is best explored in New Game+
When Alex put our Starfield review live earlier today, he liked it a lot, and I've spent the last two weeks enamored with its world much the same way. I would implore for any player to go into the game as blind as possible to get the most out of the experience - with one key exception. Not only does Starfield have a New Game+ option, unlike Bethesda releases of the past, the way it works makes it an easy recommendation for players to wait until they've reached it before truly engaging with Starfield's side content.
Why should you wait to explore until New Game+?
The answer for that question is simple; without spoiling the hows, whats and whys - Starfield adds new content in New Game+, and none of the quest progress or equipment you've found or earned in your original game will transfer over. There are a few exceptions to the rule - which we'll go over shortly - but essentially, if your plan is to go for as close to 100% completion as you can, you'd be best served waiting until the full slate of content is available to you. Until New Game+, that won't necessarily be the case.
Now, while most of what you gained from your first playthrough won't transfer over into a new one - your level, skills, and some special abilities you'll gain during the course of the main story will. This means that there is at least some benefit to lingering in your original playthrough to find as many of the unique abilities as you can before making the jump. The experience and skill points you earn along the way will be preserved too, after all.
Again, for the purposes of keeping the journey as unspoiled as possible we won't share how or why the game might change in New Game+, so just be warned! If your goal is to squeeze as much out of Starfield as possible, perhaps beelining it to New Game+ might be in your best interests.
What will, and will not, transfer over to New Game+?
Any items on your person, or otherwise in your possession, will not transfer over to New Game+. Any unique abilities, skill points or skills you've invested in will make the jump with you. There are some bonuses you'll gain when you enter New Game+, and you'll be able to choose whether to replay the main campaign or to skip it entirely. Any relationships that you may have made with characters in the previous playthrough will be reset, just like everything else. Once you've made the jump, you will be unable to return to your original save unless you explicitly kept a backup ahead of time.
Best of luck, travelers, and enjoy your time with Starfield!