How much reputation does a steam user have?

Python 3, 73 bytes

I'm sure this answer is garbage and will be beaten soon, but there's no other python answers yet

lambda x:sum(["- +".index(i[0])-1for i in x.split('\n')if i[1:4]=="rep"])

Use like this:

f = lambda x:sum(["- +".index(i[0])-1for i in x.split('\n')if i[1:4]=="rep"])
print(f("PUT INPUT HERE"))

Fetching from steam

Here's some sample code which fetches the first 100 comments from KennyS' profile and calculates his rep.

import requests
from bs4 import BeautifulSoup

# Kenny's profile as Steam ID 64
# You can adjust this to whatever numbers you want
STEAM_PROFILE_URL = "76561198024905796"
payload =  {"start" : 0, "count" : 100}
r = requests.post("http://steamcommunity.com/comment/Profile/render/{}/-1/".format(STEAM_PROFILE_URL), payload)

# Comments are html inside a json object
soup = BeautifulSoup(r.json()["comments_html"], "html.parser")

# Get raw text for every comment.
# The " ".join() strips out the newlines and tabs which are part of html
comments = [" ".join(s.text.split()) for s in soup.find_all("div", {"class" : "commentthread_comment_text"})]

calculateRep = lambda x:sum(["- +".index(i[0])-1for i in x.split('\n')if i[1:4]=="rep"])

print(calculateRep("\n".join(comments)))

Perl 5, 25 bytes

24 bytes of code + -p flag.

$\+=/^\+rep /-/^-rep /}{

Try it online!

/^\+rep / returns 1 if the line starts with +rep; /^-rep / returns 1 if the line starts with -rep (so only one of them will be one at most). We use $\ to store the result, as it is implicitly printed at the end (thanks to -p flag and those unmatched }{).

05AB1E, 18 16 17 bytes

Saved 2 bytes thanks to Okx
+1 byte due to change in spec where rep now need to be followed by a space.

|vy5£„+-S„·Ý «QÆO

Try it online!

Explanation

|v                   # for each line of input
  y5£                # get the first 4 chars of input
     „+-S„·Ý «       # push the list ['+rep ','-rep ']
              Q      # check each for equality
                     # results in either [1,0] for +rep, [0,1] for -rep or [0,0] for others
               Æ     # reduce by subtraction, gives either 1, -1 or 0
                O    # sum