What is the difference between 抽 and 拔？ We just had both these words on the same lesson and they seem very similar to me. Yet, a search about 抽和拔之間的區別 returns nothing helpful. In our book the respective example sentences were: 


  …又從另一個書架抽出來一本看看。


and


  鑰匙忘拔下來了。（title）


These two movements seem very similar to me. 

CC in pleco defines:


  抽：
  1 to draw out
  2 to pull out from in between
  3 to remove part of the whole
  4 (of certain plants) to sprout or bud
  5 to whip or thrash
  
  拔：
  1 to pull up
  2 to pull out
  3 to draw out by suction
  4 to select
  5 to pick
  6 to stand out (above level)
  7 to surpass
  8 to seize


I further examined the list of words for each character in pleco. I noticed some tendencies, but I may be wrong once again. 


  拔草，拔劍，拔牙
  seem to imply a pull against resistance.
  
  抽彩，抽空，抽功夫，抽成，抽血
  seem to imply less resistance/ more abstraction.


Is this observation coincidence? How to best describe the difference?
抽 從「手」，「由」聲，本義為引、拉。(the basic meaning is 'lead' ; 'pull')
拔 像用雙手拔取草木之形 (The original character looked like the form of pulling grass with both hand)

The basic meaning of 抽 is 'pull' (toward a direction)
The basic meaning of 拔 is to 'pluck' (拔草 pluck grass from the ground or 拔毛 pluck feather from a bird)
Since the motion of pull and pluck look similar, the two words are often used in similar way. You observation is correct, 拔 usually imply pull against resistance while 抽 doesn't imply anything in term of degree of resistance.
As you can see from your research, both character have other meanings that are not overlapped. The 'pull' meaning is overlapped but with subtle difference