The new default skin is very pretty and sharp, but unfortunately I feel it's more of a novelty and doesn't work well when a player is being challenged with a fast paced song. Furthermore, I feel that the style of the skin isn't as universally acceptable being used in maps as the previous default, but rather it's for maps with dark, cool colored backgrounds.
If the default style isn't going to be changed for over a year, I believe people should have the ability to use all the previous skins properly, as they were intended when created. (Or at least make it changeable to previous settings)
I understand that if you give players the optional choice of the previous default that they may never bother to get accustomed to the newer, sharper looking skin, but the skin is creating a problem for readability. The simplicity of the previous skin was very lovable, and probably more beginner friendly as well.
Problems that players face with the new skin include:
The hitbursts: Although they look awesome, the 300s and 100s look similar when being distracted by oncoming notes (This is probably the biggest problem I have with the skin). With the previous skin, the players could easily catch the color of the score while under a heavy barrage of circles, but these hitbursts are mostly colorless. A great improvement, however with these new hitbursts in comparison to the old ones however is how much easier it is to spot a stacked note after it.
The circles no longer enlarge: This was a really cool feature that really gave great feedback when hitting notes. The first thing I noticed when I started playing osu! was that I absolutely loved the feedback while playing this game. It's a great feature that should definitely be at least optional.
The spinner: This spinner looks sweet, but a huge problem is that there's no approach circle telling you when it's going to end (maybe this is a glitch?). There's also no bars on the side telling you when there's going to be a bonus either (not that having bars is necessarily a big deal, but the spinner is giving you no indication of how long it is). The flashing white within the spinners circle can also make it hard to see your mostly white and blue cursor on a blue spinner, but the idea of the spinner actually having a visual feedback like that for spinner bonuses is a really cool concept.
The slider: I believe that this new slider is optional if I recall, but a problem I ran into with these sliders is that they make overlapping sliders really hard. Especially when combined with the fading-in sliderticks.
The miss animation: This doesn't bother me too much, I think it's nice to see how the new skin makes all the elements seem really active, but I can see why it might bother people. Aside from it looking like the same effect as when you fail (people can adapt to that), it might confuse someone on a jumpy pattern, since the position of the X is moving from where it was previously hit. Since it might confuse some people, it is another feature that should be optional.