Hey Manu, been playing TE since 2011, almost entirely career mode. Thought I'd give some positives and critiques of the alpha so far from my own perspective.
1) The slice and dropshot are much better than in TE2013. You can actually play a Feli Lopez type character now and slice most of your backhands with the right stats. The dropshot is also really good now for catching players standing too far behind the baseline.
2) I know a lot of people seem to disagree, but I feel that the slowed down side-to-side movement is overall better for the game. It makes the 40-60 shot rallies that were common in TE 2013, less so.
3) The shot system seems to be more realistic, with badly judged shots hitting the net and such.
4) The courts and graphics look really good so far. I'm looking forward to seeing the new players with facial customisation.
1) The volleying system needs a big overhaul. It actually feels worse than TE 2013 currently. You need to make the volleys cleaner, there's too many mishit ones that just land in the middle of the court or go out. You also really need to add proper half-volleys and low-volley animations. Currently, if you run towards the net, and you aren't quite close enough, you'll be forced to hit a ground stroke which will go out. Having proper half and low volleys would solve this.
2) Return of serve needs to be made less effective, particularly on the second serve. Currently, you can stand right on the baseline and hit return winners off second serves with little problem. Same goes for the AI returning your second serves. This makes playing a serve-volleyer even harder.
3) Heavy topspin isn't as effective against backhands (particularly single-handed) as I was hoping for. My suggestion would be to reduce the efficiency of backhands on high balls in general (one-handed more than two) and add a talent that allows them to bring the backhand close to parity with the forehand in this regard (so you can model guys with strong high backhands like Djoko, Nalbandian and Guga).
4) It'd be nice to have a system where we can customise an AI player's behaviour a bit. For example, make a player who uses a lot of drop shots, or slices his backhand all the time. I think you might be looking into this already though?
Finally, I'd like to ask what some of the talents do. I haven't noticed any gameplay changes with the "ball feeling" and "early ball hit" talents. Are these implemented yet, or do they give only minor bonuses?