So a few pages ago when pre-National Dex Pokemon list was figured out there was some debate over what exactly the in-game teambuilding situation compared to Platinum was, so I've compiled a quick list of notable changes compared to that game in this aspect. This is not definitive, there may absolutely be things I'm missing, so let me know about those!
Pokemon Lost: Vaporeon, Jolteon, Flareon, Espeon, Umbreon, Tropius, Tangrowth, Yanmega, Leafeon, Glaceon, Porygon-Z, Probopass (-12)
Pokemon Gained: Pinsir, Jynx, Mew, Ursaring, Jirachi, Mismagius, Honchkrow, Purugly, Skuntank (+9)
Pre-existing options that got significantly buffed
-Every Pokemon that got Fairy type (Clefable, Mr. Mime, Azumarill, Gardevoir, Togekiss)
-Houndoom, Scyther, Swinub, Skorupi and Croagunk being obtainable MUCH earlier via Grand Underground
-Munchlax being actually feasable to use due to Grand Underground
-Swords Dance Bibarel
-Quiver Dance Wormadam
-Early Recover Shellos
-Roselia is in Zone 3 of GU, meaning that in exchange for waiting a bit you don't gotta babysit Budew
-Togepi also requires much less babysitting due to GU availability at much higher level than Cynthia's gift
Pre-existing options that got significantly nerfed
-Strength boulders cover up Gible cave, also only appears in Underground at Badge 7
Like I said, probably forgetting things on both ends, but all in all it seems like a net gain to me