1. type 1 is so much faster that any card advantage significantly avalanches into victory
I'm not a veteran of multiple formats, but I think this is very much the big reason that draw spells are prime counter targets. Note, however, that Smmenen recently had an example on MTG.com (or was it SCG? can't really remember now) where he
wouldn't counter a first-turn Ancestral with Mono-U.
When people talk about "threats" in Vintage, there actually are not so many (at least not in decks with enough draw in them for this issue to be relevant). Decks with relevant draw spells (especially decks with more than Ancestral) do not have enough threats to wait to counter them, because by then your opponent will have at least as much protection as you have counters. There are also two different types of draw in Vintage that you need to be aware of: the must-counter ones, and the other ones.
In the first category, I see Ancestral, Intuition (unless it's for AK, but who knows that beforehand?), Gifts Ungiven, and Fact or Fiction. Ancestral is important in that if you let a first turn Ancestral resolve, you might just lose that very turn because you don't know what you are up against. Later in the game, Ancestral provides the boost that is likely to push your opponent over the top to win, and it is such an easy way of catching up that you should prevent it. Intuition and Gifts Ungiven are easily capable of getting threats that are immune to counterspells (dredge cards, Recoup), and especially Gifts is dangerous here both because of a possible insta-win and the ability to tutor for more disruption/ countermagic. And while the old line "EOTFOFYL" is not heard as often anymore, it still is true in many situations.
Thirst for Knowledge is the draw spell that sits on the fence, because it is highly dependent on the board situation. With a Welder in play, you do not want to let TfK resolve. On the other hand, if you are ahead or have answers of your own (e.g. Brainstorm in response or similar), you might let TfK resolve at certain times. Same goes for Deep Analysis, which is rarely worth Forcing because that doesn't really stop it. As a Drain target, on the other hand, it is juicy. The cantrips are rarely countered, and for Skeletal Scrying it depends on how big it actually is.
Draw spells are often referred to as "threats" themselves when players talk about Vintage matches. That is evidence that draw spells really are that important. It all comes back to the power that draw spells can reveal. Best example: the Tendrils kill off a Draw-7 (which are a whole category for themselves and have special rules for countering or not).
Dozer