This is an interesting question, @msp's answer is a good elaboration and I would have posted something similar but I then looked literally at the question and thought:
"What are most sites doing by forcing you to have things like 1 number, one capital etc"?
When you think about it like that the site is likely trying to avoid you unintentionally using a password like "password" by making it more "complex" by putting in "Password1!".
In a lot of ways this is great, you're now more secure...assuming your start point was terrible. If however you have chosen "correcthorsebatterystaple" and are then forced to add "1!" or similar then I can see why the OP is asking for guidelines about whether enforcing such policies is positive or negative.
I then tried to think from a few perspectives. If me an experienced user of computers tried to use the site in question with what I felt was a secure password and was then asked to amend it for that specific site, I would be annoyed, probably use a default "1!" or similar if I felt the site was worth it, then likely get locked out a few times when using the site as I wouldn't remember the change I'd made. This is exacerbated if I ever had to reset my password and was forced "not to use the same password as before", then I'd end up with "2@" or something and forget that next time round.
However, if I was my parents and I'd originally used the password of "myname" then being forced to use "MyName123!" would possibly make things more secure, but the annoyance and issue above still exists.
When you boil it down, is a site responsible for the strength of your password? if so, as @msp mentions, length should be the criteria and likely nothing else.
Recently I also believe the status quo is that having to change your password every X days is deemed less secure (based on the increased likelihood of people writing things down) than allowing a password to stick. This puts the emphasis on the user which, frankly is where things should sit.