Here's a little thought experiment. Consider how you generate characters in any roleplaying system. What questions do you answer about your character when selecting specific features or choices?
A lot of games follow the D&D standard: Who you are (Species), who you were (Background), and who you want to be (Class).
Species tells you who (or what) your character is in purest form. It sets your baseline, usually by adjusting stats and adding a unique feature or ability. Ironically, this core decision rarely plays a factor in your character's identity and portrayal during actual play outside of the mechanical benefits and features granted.
Background gives you a few perks or bonuses that your character carries over from their previous experience. Oddly, you don't see many direct options to continue that path as part of your character's continuing development or growth, unless it fits into your new path.
Class is essentially a path of who your character wants to be. It isn't just the uniform they wear, but a progression to someone they actually want to become. Rarely does a player think "I want my character to be a level 1 blank."
And sometimes, players want to take multiple or unusual paths just to become something different than what the standard paths prescribe. However, without baseline models to define what the standards are, there is no reference point to determine the extent of their deviation.
Some people want to play the best warrior in the land. Others don't want to be called a warrior; they just want the awesome warrior stats, warrior bonuses, and warrior abilities to beat the best warrior in the land without being told how to do it.
If you look at other systems (or one you're trying to build for yourself), how do they work differently? Do they answer these three basic questions with similar options? Do they ask different questions that are more important to the game itself? What else can be used for players to show who their characters are, who they were, and who they're trying to be?
Personally, I like a system that shows me how to become a warrior in the land, and then allows me to make my own warrior without a prescription. Don't sell me an endless variety of options when there is only a couple choices that the system requires or expects me to make.
I don't like systems that change their complexity just to facilitate a sense of accomplishment or progression. A lot of these games are fun until they reach a certain level largely because the numbers and options get increasingly more complicated. My preference is for horizontal growth (improved options, bounded numbers) over vertical growth (more options, escalating numbers).
Lastly, a simple class system will suffice if the system has the flexibility and depth to allow decisions made during play to matter more than choices made long ago on your character sheet.