One formal difference to note is that categorical, but not hypothetical, imperatives allow for modus ponens, or detachment of the consequent obligation. Consider:
If you want to torture children, you should seek psychiatric help.
You want to torture children.
Therefore, you should seek psychiatric help.
That seems perfectly valid. But suppose we replace the first premise with a merely hypothetical imperative:
If you want to torture children, you should volunteer as a babysitter.
You want to torture children.
# Therefore, you should volunteer as a babysitter.
When we affirm the first premise as a mere hypothetical imperative, we mean it in a sense that does not validate such an inference. We might add, "But of course you shouldn't want to torture children, and so you shouldn't take the means to this atrocious end either."
This suggests that we can't take the conditional at face value. One natural thought is that the hypothetical imperative is doing nothing but highlighting a means-end relationship. But as Sidgwick noted, this can't be quite right: there's some added element of normativity here, tricky though it may be to pin down. A more promising suggestion is to interpret the 'should' of the hypothetical imperative as really taking wide scope (i.e., "you should: either not want to torture children, or volunteer as a babysitter"). But does that imply that, if you fail to satisfy the first disjunct of the requirement, there's something to be said for the sadist's deciding to satisfy the second disjunct instead? One would hope not. Perhaps the best solution here is to interpret the antecedent of the hypothetical imperative as itself implicitly normative; something like:
If you rightly want to torture children, you should volunteer as a babysitter.
Either way, the crucial point is that hypothetical imperatives don't necessarily have normative force even when the agent possesses the desire in question. Even if you have the desire in question, you may still disregard it on the grounds that it is misguided and lacking in normative authority. (This is a point Velleman makes nicely in his 'Brief Introduction to Kantain Ethics'.) Hypothetical imperatives are thus best understood as depending, not merely on your having certain desires, but rather on those desires having normative authority. The hypothetical imperative "if you desire X then you should Y" really means something like, "supposing that X is a worthy goal, you should Y."
Hypothetical imperatives thus reveal patterns of normative inheritance. But their highlighted 'means' can't inherit normative status unless the 'end' in question had prior normative worth. A view on which there are only hypothetical imperatives is effectively a form of normative nihilism -- no more productive than an irrigation system without any water to flow through it.
This observation puts advocates of Humean "desire-based views" in a bind. They had hoped to achieve a kind of unmysterious, uncontroversial normativity. But so long as they merely affirm hypothetical imperatives, they're stuck with a flimsily dressed up nihilism. Alternatively, if they say that we're categorically required to act on our present desires, then their view is absurd. ("You really mean that the self-controlled sadist is making a mistake when he disregards his urge to torture people!?") Once we think there are genuine reasons to do some things rather than others, we may as well pick a more plausible theory of what it is that we have reason to do.