We play medium to high optimization SR4 games and I use the following guidelines for expected DP for a given skill level:
4: You're not completely inept in this area but you're pretty close. The lowest ranks of 'normal' human ability. If you have less than this you are 'retarded'.
8: You certainly aren't a professional, but you've done it before a few times and are quite competent at normal tasks in this area. You could even try and pretend to be a professional, with some luck. More likely you use this skill as a backup in case no one better is around to do the job, or it's part of a set of related skills you've gotten broad expertise in.
12: You are a competent, trained professional. One of the faceless masses.
15: You might get scouted by a mega-corp as a promising up-and-coming specialist in this area.
18: You are elite. You are better than the others, the lesser metahumans who think they know the ways of your chosen skill, but they are all blind, blind to the true Way of your speciality which you have immersed yourself in. But there are others like you, and you are content to confine yourself to their affairs, their society, and leave the fools to their blindness.
21: Even your enemies are hesitant to kill you, because then they could no longer hire your services, and your services are in quite some demand. Expect to be assaulted with non-lethal capture attempts on a regular basis. Be very careful who you trust. You wouldn't want to get 'extracted'. You can work practically anywhere you like, if you can avoid getting kidnapped.
30: Dragons fear you. You are the best in the world at what you do. You skill defines you, it creates for you your way of life and just as surely cuts you off from what little it cannot accomplish. And with your prodigious ability that is very little indeed.
36: You aren't just the best, you compete with the best that ever were. People don't really think of you as a demihuman. This probably includes you. Expect to push the boundaries of the possible. Cry out when you fail.
These, of course, vary widely by discipline. Damage resistance rolls in specific areas, for example, are easy to raise far above these numbers (about X3 at the high-end) while general medical ability is actually kind of hard to get this high (about 20% lower). These numbers and the scale above are for our games, which use the Core Rules, Runner's Companion, Arsenal, Augmentation, Unwired, and Street Magic exclusively. Additional material might raise the numbers slightly (and significantly in areas below this curve that get more material), while less material will significantly lower the numbers for specific subsets of characters.
These numbers turn into the thresholds below:
1-2 : Extremely simple. No training required.
3: Routine task for an intern
4: Routine task for a professional
5: Challenging. Most people in the field would briefly struggle with this, but would succeed after trying a few times.
6: This kind of success goes on the CV/in the portfolio of an average professional. It's the kind of thing the average expert is proud of having done.
8: The average professional, given time and resources, might not be able to do this anyways.
10: Just barely 'Impossible' in the minds of the common folk. The bleeding edge, the pinnacle of the art.
14: Inventing/Discovering some minor new thing allowing significant but not revolutionary advances in a field.
16: Revolutionary advancement.
20: The work of gods, not men.