Another Python solution, at 247 chars. The actual code is a single line of highly dense Python line of 134 chars that computes the whole thing in a single expression.
x=3;y=2;W="and is the as of to or in for by an be may has can its".split()
from itertools import groupby as gb
d=dict((w,l)for w,l in((w,len(list(g)))for w,g in
gb(sorted(open("c.txt").read().lower().split())))
if l>x and len(w)>y and w not in W)
A much longer version with plenty of comments for you reading pleasure:
# High and low count boundaries.
x = 3
y = 2
# Common words string split into a list by spaces.
Words = "and is the as of to or in for by an be may has can its".split()
# A special function that groups similar strings in a list into a
# (string, grouper) pairs. Grouper is a generator of occurences (see below).
from itertools import groupby
# Reads the entire file, converts it to lower case and splits on whitespace
# to create a list of words
sortedWords = sorted(open("c.txt").read().lower().split())
# Using the groupby function, groups similar words together.
# Since grouper is a generator of occurences we need to use len(list(grouper))
# to get the word count by first converting the generator to a list and then
# getting the length of the list.
wordCounts = ((word, len(list(grouper))) for word, grouper in groupby(sortedWords))
# Filters the words by number of occurences and common words using yet another
# list comprehension.
filteredWordCounts = ((word, count) for word, count in wordCounts if word not in Words and count > x and len(word) > y)
# Creates a dictionary from the list of tuples.
result = dict(filteredWordCounts)
print result
The main trick here is using the itertools.groupby function to count the occurrences on a sorted list. Don't know if it really saves characters, but it does allow all the processing to happen in a single expression.
Results:
{'function': 4, 'operators': 4, 'declarations': 4, 'which': 4, 'statement': 5}
