1. Limited Movesets are so that the game is balanced and doesn't have a ton of Brave-to-HP combos or HP wall slams (that caused up to 9999 + 9999 damage in Duodecim). This game has a lot less defensive options than the previous game too, as EX Burst was the only way to escape combos outside of blocking/perfect defense. The reason for that is to prevent turtling play.
2. HP attacks specifically are meant to keep things tactical so you have different uses for different HP attacks. Some are ultimately better than others in most situations, but it also depends on your role/playstyle. Wide stages are better suited to long-range HP attacks, while small stages can use close range HP attacks that do more damage per hit. If players had 10 HP attacks in their arsenal things would be overly chaotic in a team-based setting.
3. The Brave Pool was technically supposed to be used in conjunction with Base-HP count. The game has an HP cap now, so really you only need like 3,700 Brave to one-shot anyone now. I wish they'd fix this too, but because it's 3 on 3 they can't give individual characters high stats anymore.
4. Stages no longer drain Brave either, only certain debuffs do. The reason being that they wanted to focus on Team Play over single player battles. Adding extra stage effects would probably make things more convoluted (like hiding behind walls and spamming HP attacks that go through walls).
This game is okay for an Arena game, but the emphasis is ON multiplayer, not single player... so if you play by yourself it's not going to be AS fun. This is a problem that plagues most MOBA Arena games.
This is pretty much the exact same reaction people had when Guilty Gear XX became Guilty Gear Isuka... Though, for what it's worth, Isuka was a lot better constructed as it used #Reload for it's gameplay foundation as opposed to this game which has no really good foundation to begin with.