I am a software developer... I haven't been given any tasks where I could really make a difference
The most common mistakes software developers make on their resume is focusing purely on their hard skills. So you've basically assumed you didn't achieve anything because there isn't a clear hard skill used to achieve it.
Allow me to break down what you said and try to express each as an achievement, and guess what. It's all soft skill achievements and you should be proud of yourself. It's a real shame you don't see it this way.
working in a team of software developers. Have been promoted from junior level recently,
"Joined the software development team as a junior level developer, and was later rewarded with a promotion to intermediate developer".
You could qualify the above with numbers and ask your supervisor why you were promoted. They'll have something positive to say about it.
What I do in my job is implementation of new functionality for our software according to the specification. It's the architect that designs functionality and all I can do is come up with my implementation.
"Earned the trust of the senior architect to implement his/her design specifications as new functionality in our software."
How many specifications have you implemented so far? How many architects have you received specs from? How long does it take you to implement a single spec?
These are numbers that can be added.
I haven't been given any tasks where I could really make a difference, like design a significant piece of functionality or invetigate and fix serious issues that affect software globally.
"Set a personal goal of becoming a senior architect where I could work on new designs and investigate serious issues. Getting promoted was my first step towards that goal."
How many tasks did you complete before you were promoted? Did your promotion come with a raise, did you receive any new responsibilities, how much more work do you do now after being promoted? By what factor are specs more difficult after being promoted?
People absorb achievements easier when there are numbers. Achievements are subjective but numbers help turn them into factual claims.
So what I do is I do my job. I do it well, but that's it. My only achievements are doing my job well.
You need to think of achievements that benefit the employer, and not yourself. You seem to be focused on "my achievements" rather than "achievements for others".
I can tell you, that if you did your job well, then you benefitted your employer and helped them to achieve more. It's the definition of what an achievement is. It's to do a good job and that's why they hired you.
At the end of the day, if the company pays me money to do what I do, it must be BENEFICIAL to them.
YES! You hit the nail on the head, and then walked away from it.
BENEFICIAL to them is exactly what you want to share, and YES they spent money on you. That's the entire point here. It's not about how it benefited you, but how it benefited your employer, because at the end of the day you're asking someone else to spend money on you. That's what job seeking is.
It's all about money, and you want your resume to say that past employers who spent money on you saw a financial return (as money in another form). The idea is that your past successes will translate into new successes at a new job.
In other words, simply doing my job well is in itself a benefit to the company, or else they would let me go, wouldn't they?
The discussion here is your resume, and how achievements play a role in getting you an interview. What you just said, just explains why they don't fire you. It's a different discussion.
I can see how project managers and sales people can do things like raise sales by 30 percent and improve processes in their company but I don't see how a regular level software developer can make any significant difference in a large company. And that's who I am. So how can I turn my job description into achievements in this kind of situation?
I absolutely agree with you here, and the problem has more to do with who's giving resume advice than questioning the advice itself. The majority of resume columns you read talk about sales people, and their achievements because they represent the majority of corporate workers. They are the broadest audience, and if you're going to write articles about resumes. You write for them.
That leaves a junior developer feeling small and insignificant in the bigger corporate picture.
Ask yourself: "Who decides who gets an interview, and will my achievements compel them to call me?"
The above is an important point here. A resume is relative to who is reading it. If the HR person makes interview decisions, then you need to compel that person to call you, but if the senior developer decides who gets interviewed. Well, you need to compel them differently.
You want your resume to communicate that your hard skills and soft skills uniquely qualify you for the achievements you made before, and if they hire you that you'll continue to achieve similar results for them.
I can't tell you what your achievements are, but you have them and that's the point of this answer.
If you can ask your boss for some personal advise. Ask him what he thinks your achievements are, and I think you'll be surprised by his answer.