Matrix is awesome, it just desperately needs better frontend apps. Element is still pretty bad, and other alternatives are often riddled with bugs or missing features
It's for the most part very minor changes, the biggest change is the slider accuracy and how sliderballs move. Most other stuff is very minor that you'd mostly notice with a lot of hours in osu! stable
One of the new changes in osu!lazer is that you also need to have accurate rhythm for sliders. Turning on classic mod turns this off. In the mod settings you can turn it back on again, should be called "Sliderhead accuracy" or something along those lines