I'm coming from maining Kirby in Sm4sh and am having a pretty good time with Ultimate.
In terms of playing Kirby, I like how a couple of his short, bread-and-butter combos involving tilts and aerials are relatively the same, as it made transitioning to him in Ultimate a lot easier. The only significant change that I've seen is that his down-smash launches the opponent at a lower angle, which makes it an abysmal kill move in comparison to Sm4sh's down-smash. Other than that, I don't really have any complaints.
World of Light is pretty fun too. Sure, there isn't much of a plot to it, but at least it's something, especially something superior to the mediocre Smash Tour on Sm4sh WiiU. My only complaint gameplay-wise it is that some of the challenges are unfairly difficult, such as the infamous Pauline spirit. However, they're not required to beat the game, so it's not that much of a hindrance to the experience to me.
Despite most of my friends who got the game before me telling me how bad the Online mode is, most of my experiences with it have been pretty good. I like how GSP is being used as a means of showing a player's relative skill level, giving it more of a meaning outside of the game's extra modes, unlike Sm4sh. There have only been a few instances where I've been in one-on-ones with people who turn items on or use gimmicky stages in their rule sets. As a result, my experiences of the Online mode are much more positive (but also a little negative, as I tend to lose a lot more than I'd like to :^) ). I'm also pretty intrigued by Elite Smash. I'm pretty sure it's harder to get it now, especially since I got the game on Christmas, but I'm definitely looking forward to unlocking it soon enough.
I'm definitely astonished by the size of the game's roster, and pretty excited to play with some of the characters that are either new or haven't been around since the older titles. Brawl was my first Smash game, so I never really got to experience playing with some of the characters that were exclusive to Melee. Now that every character from all the installments are playable, I'd like to try some of them out and improve with them, especially Pichu. I also really like some of the newcomers. As a fan of Splatoon, I was pretty excited to see the Inklings, as they have an arsenal of weapons in their own games that made them perfect contenders for Smash in my opinion. I'm also interested in trying out Isabelle and Incineroar, as I enjoyed playing Villager casually in Sm4sh and want to elaborate upon and refine some things I've done when playing Incineroar on occasion. Overall, I really like the large selection of fighters Ultimate has to offer.
I believe I will be spending a lot of time playing this game. For a multiplayer-based game, it's pretty fun to play by myself or with other people. The vast and soon-growing selection of fighters makes Sm4sh's roster seem very small. I've only had a few complaints on this game so far, and I doubt that amount of complaints will grow any time soon. I'm pretty sure this will become my new favorite Smash game soon enough.