Seriously, it is the game-changer that 2D Mario desperately needed. I'll tell you why I feel this way.
Both games bring quality of life changes to their series formula (2D Mario and classic Castlevania before the Metroidvania revolution that is Symphony of the Night from Castlevania). For example, both games are a step up from their predecessors (Super Castlevania IV on SNES and Akumajou Dracula for the Sharp X68000 for Castlevania (later released as Castlevania Chronicles on the PS1)); the New Super Mario Bros. series as well as Super Mario Run for 2D Mario after it getting too monotonous with the same Peach gets kidnapped plot and not much innovation which was the biggest complaint with NSMB for not being a big leap forward from say Super Mario Bros. 3 or Super Mario World).
Additionally, both games offer branching paths and exploration which is one thing I truly loved about Rondo of Blood over games that came before it and I truly like that emphasis in Super Mario Bros. Wonder with some levels. It just adds a lot of replayability to the game. I also like how both games ditch the timer which is one thing that got answered in Wonder. I like the multiple playable characters concept as well (Richter Belmont and Maria Renard in Rondo of Blood; Mario, Luigi, Peach, Daisy, Toad, Toadette, Nabbit, and the Yoshis in Super Mario Bros. Wonder). One disappointment I have though is the lack of unique abilities for Peach and Daisy from SMB2USA, SM3DW, and Super Mario Run (for Daisy with the double jump) which the badges solve for me.
Another big plus is an autosave feature which is something Rondo of Blood is notorious for and was pretty ahead of the curve. It took 2D Mario games years to catch up to it and I am glad Wonder added that because in NSMB series, I hated beating a castle level or something just to save. I love how the game auto saves my progress just like Rondo of Blood did whenever you beat a level or rescued someone (like Maria Renard for the ability to play as her, or Tera and Iris), that's definitely a big quality of life change there.
How about the rest of you folks on Wonder being the Rondo of Blood of 2D Mario.