Wow, I had to check twice to make sure this OP wasn't posed by me and I had forgotten I did it. Great question, in other words!
Michael Bluth and Jim from The Office US are the two examples that come to my mind. They are just normal guys wanting to get along in unusual circumstances. They elicit audience empathy and don't distract from the other, bigger characters.
However, there's always the worry that they're just not... interesting enough.
This blog post might help:
http://sitcomgeek.blogspot.com/2011/12/central-character-needs-work.html
I went through this myself recently. The two main characters were boyfriend and girlfriend working together at their restaurant. The boyfriend probably wasn't distinct enough. So I decided that they didn't have to work together. In Ep 1 they find out that's not going to work. They still live together and spend time together so really there's no actual need for them to work together. The boyfriend's constant search for a new career path gives him more distinct needs. Plus, I think the difference between the two characters central needs (girlfriend: be successful, make money. Boyfriend: have fun, make the most of youth) can become the central theme of my show. Don't know if that's useful to you. Probably not. But I think you'll agree that Michael's needs are always very opposed to those of his family (remember how quickly they all sold off company shares). And that whole thing with Jim's sporting goods company set him at odds with his job and his wife.
I think another key aspect is likability. To be a sympathetic lead you'll probably have to achieve that. So, they're good at something, but also vulnerable in other areas.