To try to explain why people are disappointed...
Remember when, during E3 2013, Nintendo showed off its then brand new 3D Mario game, the first HD 3D Mario? Remember how at first everyone seemed to hate it? Even today I see plenty of people on Vchartz complain about how it wasn't a 'true 3D Mario.' Why did it get that hate then and still gets those complaints today? Because it was an HD console game that was using the style and formula of a handheld game. People were expecting something ambitious and mindblowing, and they didn't get it. And for some, the fact that the game was a blast to play still doesn't matter and they are still upset that the Wii U never got a 'true 3D Mario.'
Something similar is happening now. This is not just the first home console Pokemon, but it is releasing on a console that is more powerful than the PS3 and Wii U. This is a console that has seen BOTW, Skyrim, and Xenoblade 2. Gamefreak has already shown a hint of what this new Pokemon experience could be like with Lets Go and having Pokemon wandering on the world map. It was always a given that the graphics wouldn't be as good as Zelda or Xenoblade on the Switch, but this still offered an opportunity to expand on the world and adventure of a Pokemon game like never before and reintroduce that sense of wonder by looking out at a field and seeing herds of Pokemon roaming around or a legendary fly off into the distance. And this trailer has all but killed those hopes. The world will look nice, but the models remain pretty much the same as those used in 2013 on a system that could only do 240P, the primary design of the game appears to remain the same as it was in 1996 on an 8-bit system that couldn't even handle 3 colors, with any changes being pretty minor, and the chance to rekindle that sense of wonder fans got the first time they ventured into Veridian Forest or found Articuno 20 years ago has been squandered by the decision not to have Pokemon on the overworld, which is a decision that is going backwards from the last spinoff.
Many people are happy with what amounts to a 3DS game with HD environments. But others wanted that experience of feeling like they were truly in a world of Pokemon, to be surprised not by what attacks you in a separate screen, but by what walks by, flies by, swims by, scurries about their feet or towers over them as they explore. This isn't something that is so technologically demanding. Games have been doing it for years. The first Xenoblade did it on the Wii and New3DS. Monster Hunter was doing it on the PS2, Wii, and 3DS. Not having it here is a big letdown for anyone who wanted that feeling of wonder that even Pokemon Go captured.