In terms of special abilities unique to your type (Nano, Glaive, Jack), you can't do those things unless you actually earn the ability. So a Glaive can't cast esoteries no matter how much Effort he uses.
The idea in Numenera is that anybody can attempt a task -- like picking a lock, persuading a guard, or jumping across a chasm. The difficulty of the task is determined by the task itself, not by the character attempting it. So jumping across a 6' chasm is (to make up a number) a Difficulty 3 Task. That difficulty determines what target number you have to roll to succeed. If you are trained in jumping, it decreases the difficulty. If you apply Effort, it decreases it even more.
So a trained jumper with low Might might have the same chance of success as a very strong, untrained person who spends their Might on the task. Additionally, a trained jumper with high Might would not only have a higher chance of success, but would be capable of jumps that the other two builds couldn't even try (or they could, but they'd fail). And an untrained character with low Might could maybe tackle an easy jump (low Might doesn't mean no Might), but any serious jump would probably be too difficult even with the Effort he could apply.
So it doesn't eliminate choice at all, but it does suggest your stat pools (Might, Speed, Intellect) are slightly more important than what skills you train in. Monte is fond of saying that Tier 1 characters are actually quite powerful already, unlike level 1 characters in most games. Effort is part of that.
And this is just talking about generic "difficult tasks." As I said in the first paragraph, there are lots of things that will make your character unique beyond stats and skills. Your type, descriptor, and focus (mostly type and focus) give you bonuses and abilities unavailable elsewhere.