Hi there! And thanks for the feedback :)

> I have big problems getting used to the controls.

At first, I had some problems, too. But for me, after hours of playing, they resolved themselves. There is no way to change back to the old controls; as the game now heavily depends on the mouse and proper steering of the clonk, it would be nearly impossible to simulate anything close to the old controls. It is also undesirable, to maintain two different control styles. We had that in Clonk Rage and one or the other got often overlooked and if not, it was twice the effort.
You can set the movement controls back the numpad of course, if that's something more convenient to you. WASD just was the first choice at hand as many and more other games nowadays utilise these. The new controls are easier for anyone not coming from an older Clonk title. If you still got trouble, you might want to try Clonk Rage with the Jump'n'Run controls first. Transition from Clonk 4 to OC might be even harder since you left out 3 whole games!

> overall graphics style

We are currently in the process of a graphics overhaul that will hopefully lead to more contrast in textures. It wasn't ready for the current release though and the people responsible haven't given any live sign since a couple of month ago.
I agree on what you said about the menu backgrounds.

> Did I just not find it, or was the leveling system completly dropped?

It is dropped. Again a maintenance thing. With Clonk Rage, online play in Clonk became more and more popular. But in order to play online, you had to do several hours of offline playing in order to get your clonk(s) to full health and being able to climb. Without climbing, your clonks often remained useless in competitive play. Also, cheating was a bit of a problem as you could easily edit your clonk's profile. Thus in CR the fair crew button was introduced. Activating it always gave every player the same characteristics for the clonks, making competitive games far better. Soon afterwards, progression in climbing skills was abandoned as this was a huge drawbacks for any newcomer and veteran player as well. As soon as all of your 'good' clonks were depleted, you were left with the useless ones.
Training your clonks became more and more of an offline feature and less important.

It's always great to see old and new players rediscovering the game and taking their time to write good feedback :)

I'll try to explain a bit, why certain things were changed and what the motivation behind those changes was.

The controls first: even though experienced players mostly had only little problems with the controls, new players got frustrated very quickly and had a hard time learning the controls. Clonk never was a "game for everyone" and we think that this was to a good part due to the lacking controls.
Not only were they pretty unintuitive, because players could not compare them to any other game they played, they were also pretty tricky and often required weird combinations of buttons. One example is that the old Clonk titles basically used double-dig for nearly everything. Ever tried to chop a tree while carrying a construction-kit or something like that? Well, with the old controls that simply wasn't possible.
Because the problem was known not only to us but also to the original Clonk father (Matthes Bender), he and the ClonkRage development team introduced a reworked control scheme in ClonkRage a few years ago. But that rework was highly limited by compatibility issues: you could not do any real changes without breaking all existing objects.
And even later on those controls turned out to be more hassle for developers, because certain work had to be done twice.

In OpenClonk we now had the real cool opportunity that we could re-create the complete game controls without having to worry about breaking existing stuff. So we sat down and talked a long time about how the controls should work and what we thought was best for the players. That process is not finished yet btw! Control changes are going to come at some point in the future.
But we ultimately decided on a control scheme that would be a lot more intuitive - nearly every player has already played games with WASD+Mouse. And not only would it benefit new players, but it requiring the mouse would also give us, as the developers, new possibilities for objects, interaction and gameplay.
For example aiming with the rifle in the ClonkRage Western-Pack was pretty awkward - you had dozens of angles and aiming up and down one angle requiring one button press. So, to actually hit stuff you had to hammer your buttons like a mad badger.

I believe the control changes turned out to be a good decision - in OpenClonk you can actually just give the game to a new player and have them play the game instead of trying to work out how to dig a tunnel for half an hour.
It might need a bit of adaption by veteran players, like you, but I think that Clonk players were always pretty good in adapting to new things and situations so I believe that it shouldn't take to long to get used to the new controls :)

To the leveling system: as Clonkonaut already pointed out it eventually turned out to be a single-player feature. Not only did it allow players to "cheat", but it also gave players unfair advantages or disadvantages in online play. It even made designing single-player scenarios harder, because a scenario that was of moderate difficulty for an experienced player could be impossible for a new player, just because their Clonks could not climb - quite unfair if you ask me! I am not saying that it was a bad feature for single player or local or just peaceful settlement games! But for now we dropped the feature because it would require maintenance and work. Work that was spent doing other, more important, features first. Maybe it will come back in some way in the future. Possibly as a user-created mod. Who knows. But right now there are no plans by us to put it into the game again.

to the textures and graphics:
That's actually a more or less ongoing discussion among the developers, too. There will hopefully be some changes to that.