Ple-Sen
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Leveling up should not be a core element, and a new character should be weaker than a fully leveled one, say, two to three times weaker, no more.
2. The main drawback of the class system is that characters of the same class are often very similar. This can be solved by making the system more complex and extensive, but in the case of experience points and manual calculations, we can hardly afford it.
Plus, as Ment has already noted, the concept often doesn't fit into the class.