What level of proficiency do you consider to be intermediate excel skills

I agree with you that the more I know, the more I see I don't know. I consider myself an intermediate/high intermediate user. I tend to use a lot of functions such as vlookup and sumif,. I also would recognize the ability to build, recreate, link, and combine worksheets. Also, not that size is always a true indicator of skill, I am constantly building and maintain workbooks of 20+ tab in order to manipulate and analyze data.
On a scale of 1-10, I usually rate myself a 7 (and sometimes lower depending what issues that have recently humbled me!).
What I have always found interesting is what ratings I get when I interview job candidates. I have had candidates tell me they are 9 or 10, but when asked specific questions, they turn out to be a 4 or 5, at best.

Here is a non-exhaustive list of skills that I would consider in the intermediate range:
Proper usage of IF, VLOOKUP, LEN, LEFT, RIGHT, MID, DATE, TEXT functions
Full command of filters
Usage of simple conditional formats (usually to highlight error checks)
Chart creation and ability to manipulate axis as desired
A basic understanding of Excel's data model (values vs presentation, dates are represented by numbers)

Here are some skills that tend to put a user in the "advanced" range:
Chaining together various functions to create a complex formula
Usage of more complicated functions (SUMPRODUCT, INDEX/MATCH, array formulas)
Pivot tables
Creation/editing of macros beyond just the recorder
Proper usage of named ranges (and named range definitions that include formulas)
Strong ability to identify and troubleshoot error values (including usage of formula auditing to hunt down the root cause)

As Wayne mentioned, there are so many features of Excel and most people don't need to use all of them for any single job or assignment. So there are many skills that might be considered intermediate or even basic that someone who is otherwise advanced may have never used.