Your system makes picking up early levels of a class very cheap. This encourages dips which are already quite viable in the current system even further. If it is goping to cost you several thousand XP to level up again, spending an encounter or two's worth of XP to pick up Second Wind, Action Surge, and maybe even a better crit range or superiority dice is going to be well worth it.
Or picking up Expertise cheaply: The classes that get it often value it quite highly; often above the level-dependent aspects of the class. Under your system, a couple of levels dip to pick up expertise is going to be of minimal cost compared to going up a level in your primary class.
Giving much easier access to what the expert classes may regard as the prime aspect of their class to everyone is going to devalue those classes.
As pointed out, this system encourages the plucking of the low-hanging fruit that is already a common reason for multiclassing, just makes the cost of it even less impactful.