title_body: How big of a difference is there between a pressurised and non-pressurised portafilter?
upvoted_answer: Pressurized portafilters either restrict water flow or require a certain buildup of pressure before the espresso spills out the bottom of the portafilter. 
  
  So there are several problems with that taste-wise in espresso. The resistance is now not in the ground coffee itself, but below it. Additionally, if your coffee is ground finely and uniformly in a good espresso grinder (as one would for a non-pressurized portafilter), this sort of portafilter will result in too much resistance and thus a slow, over-extracted shot. It can also make your crema look a bit odd because it can emulsify some of the oils in your extracted shot (depending on the design of the specific portafilter).
  
  Essentially the requisite grind size to produce acceptable espresso using a pressurized portafilter is much larger than that of optimal or delicious espresso.
  
  This sort of portafilter does have its upsides, though. If you don't have access to a quality espresso grinder, and you are using either store-ground beans or imprecisely/more coarsely ground beans, a pressurized portafilter can make acceptable espresso from them whereas the results in a standard portafilter would be very watery and underextracted.