Question: If you have a steak medium to rare, such that it's still pink in the middle, is it kosher? At what point is meat cooked *enough*?

Answer: 
What Monica answered above, that it is 'meat juice'. To find out in much greater detail about this, see [here](http://www.diaspora.org.il/Jewish_Law/on-line/05-milica.htm):

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> 1. No matter what the source of the issur, the Ran asks: We clearly observe that blood continues to come out of meat even after the salting process is over. Indeed this blood is sometimes even redder than the blood that was originally expelled. Why do we permit this meat after the required time of salting, even though there blood still appears to be inside?
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> He gives two explanations :
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> A. The liquid which comes out of the meat after the period of salting is not blood but is called mohul (juice). The Gemora refers to it as chamra boser (the wine of the meat). In other words, although it may have the appearance of blood, it is not in reality blood that is ossur at all.
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> B. The issur of blood that comes out through the salting process is only forbidden Rabbinically, and the Rabbis limited their prohibition to blood that comes out of the meat during the period of salting . After that time, anything that comes out of the meat, although it may be blood, it is not within the Rabbinical enactment. Therefore it is blood of heter and not of issur.
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This, based on the gemara in Chullin 113b.
Raw, unsalted meat is completely kosher. Just rinse and eat. Shulchan Aruch Yoreh Deah siman 67 siff 2.

EDIT. The Shach in siman 76 s.k. 2 also point out that unsalted roasted meat even if only rare is also permitted to eat.
The point of my original answer was to dispel any misconceptions about blood, but this edit will answer the assumed question more directly as pointed out in the comments. 

One more point of clarification as pertaining to cooked, not roasted meat, is after salting the red juice is not considered blood,but called mohel, as pointed out by Monica. This is the Ashkenazi law lichatchila. For Sfardim however, if possible they should be putting the raw salted and washed meat into boiling, not warm or cold, water in order to seal in the red juice which the Rambam did in fact consider blood, and the Shulchan Aruch says to take into account, when possible. See siman 69 siff 19. This is process is called chalita. 

I don't know any Sfardim who actually do this, whether at home or in restaurants (though some Teimanim do). So for all Acheinu bnei Hamizrach please comment or quote any modern day rulings, but this is what the Shulchan Aruch says and some people reading this question and these answers might find this interesting.