The AI is definitely far worse than it was in 2. It used to be that on the highest difficulty level would hardly ever give you an opening and GI everything you do. On this game, it can be challenging, but it's far easier than 2. Furthermore, the difficulty only affects VS. play against the AI. Arcade, Stoy, and Tower of Lost Souls are unaffected, and ironically I could do better in some of the challenges if the AI didn't wait like 5 seconds before attacking.
Personally, I was a big fan of soul charging, but I like this new soul system better. The idea of gradually wearing down turtle players, even though that includes me, feels nice. I almost never get a critical finish, though, except for Yoda. That guy's so bad that everyone is a good matchup against him, and yet everybody bans him from tournament.
Tower of Lost Souls is going to give you lots of work to do. It's got plenty of challenges inside it, such as never missing an attack, performing a 10+ combo (Darth Vader is my friend), and my personal impossibility, performing 5 Just Impacts. It'll also challenge you to make customized characters with the most appropriate stats and effects as you go through it. Oh yeah, and Algol at the very end of it really lives up to what fighting game boss characters do. ;)
For tournament play, Yoda and Algol are banned, while Vader and his apprentice Starkiller are not for the National tournament. Many places will also ban all Star Wars characters. I'm firmly against any of those bans, but the decision has already been made. You'll also encounter some bonus characters halfway through everyone's story modes who are identical to existing characters except for their reach. They are also banned and I have no objection. Regardless, I urge you to practice against all characters, including the banned ones, as more experience isn't bad here. I'd say you shouldn't be afraid of them, but you sound like the type of person that would welcome the challenge, anyway.