All of the work being put into Prism, especially all the mechanics imported from future generations. Abilities are in there, and cries of Pokemon as late as Generation VI are imported with accuracy beyond what was expected from the Game Boy Color's sound capabilities. Full character creation was also implemented, with front and back sprites and adjustable outfits and colors. Most amazingly however, after the leak of Prism it was discovered that the Prism file amounted to a mere 2MB, meaning that even with all the additions and added sound files, the game was still roughly the size of Crystal memory-wise and thus could run on a physical Game Boy Color cart without issue (as proven by multiple fans within hours of the leak).
The leak of Pokémon Prism mere days after its cease-and-desist-ordered cancelation caused great excitement and virtual Hail-Mary's amongst fans on 4chan's /v/ and /vp/ boards and the Prism subreddit on Reddit.
Base-Breaking Character: The Fakemon Legendaries in Prism. People either think they're well above usual Fakemon standards as far as hacks go and don't look too far off from regular Pokemon, or they get accusations of falling into the same cliches Fakemon use and/or don't look like Pokemon to begin with.
While the quality of Brown had few doubting that Prism would be a solid production, the sheer scope of the changes and the vast amount of new features Prism packed in resulted in thoroughly-impressed adulations from players after the leak.