According to Blizzard's blog post, new players are those who are making their accounts post-Overwatch 2 launch and aren't those who already have an account in Overwatch.
New players will first begin with a restricted pool of modes and heroes. In the first half of Overwatch 2's First Time User Experience, they'll unlock all the modes. In the second half, players will progressively obtain original Overwatch heroes. It will take 100 matches to unlock the full original roster.
The game’s 1.03 patch will increase the amount of experience needed to level up a single character between levels 3 and 15, as the developers found that "players could get to level 15 of the mastery track too quickly." MultiVersus fans reacted to the change over on the game’s subreddit with memes and long threads about whether or not this is a positive change. One of the suggestions a handful of users think would help the experience is the addition of more rewards after level 15. Another user suggested making a character’s gameplay related perks complete at level 10 and make everything above 10 cosmetic or currency.