Most classless point-buy systems, though I can respect them, offer me no visceral satisfaction when creating a character because there's only one obvious way to create any given concept. Want to be a swashbuckler? Get a high Dexterity, crank up your Rapier skill, and select swashbuckler-related Advantages. Breezily simple, and ideal for many purposes, but in some perverse fashion, I would rather feel my hands sink into clay when building a character rather than swimming through air.
In the opposite fashion, old-school D&D-style games are frustrating because other than the handful of concepts the game supports, there's no way to build most characters. Want a swashbuckler? Ask your DM to carve out exceptions for you because otherwise the rules say you'll be sub-optimal to the point of total ineffectiveness.
By contrast, robust class systems (and a few robust skill-and-power systems) offer multiple routes to a single core concept. In D&D3/3.5/Pathfinder, you want a swashbuckler? Right out of the core book, you've got the obvious choice (a rogue), rangers, certain fighter builds, bards, maybe one or two other classes if you rope in supplements. You've got a system that pushes back and gives you chewy mechanical ways to build different kinds of swashbucklers, or dragon wizards, or pyromancers, or whatever nerdy stuff you want.
(One of my frustrations with the monk when I ran D&D3 was its lack of conceptual flexibility. The strict, limited power progression meant that there was only one concept associated with the monk class: the "monk" concept.)
In creating Genius: The Transgression, this goal of mine reaches its deranged and borderline-ridiculous zenith, where for almost any given mad scientist concept, you can find a home in the game's 5x2 structure (ten groups total, built along two axes, so all characters belong to a "job" group and a "personality" group). And each option gives you a bit of mechanical "pushback" to your concept and a lot of narrative and conceptual pushback. If you want to play a "plant scientist," the obvious choice is the Progenitor Foundation, but less obvious choices create weird new juxtapositions that are inherently interesting. What about a Direct Plant scientist? King of the plant people! What about a Navigator (the "combat" Foundation)? A knight of thorns? Spore guns and vine grapple-hooks? Likewise, the personality axis yields strange, chewy results that enrich whatever concept you've envisioned. Genius' classes don't just support your core concept, they distort it through a prism of unusual assumptions that, I hope, gives you more ideas than you started out with.