I didnt write my own letter, but my D.O. recommender mentioned when I last talked to him (a WHILE ago) that he had sent my letter and it was "almost 2 pages, and I spoke pretty highly of you. Dont believe in putting anything negative in just for the sake of it". I havent seen the letter, but I was surprised. 2 pages is long but I worked with him pretty closely so maybe he was just doing what you said, being concise & specific. Also, I think he mentioned this was the first time he had written an LOR, so he may have gotten carried away. Who knows.
Also, I am glad at what he said. I hate it when I hear people talk about LORs and that there has to be something in there highlighting bad points, and how the applicant works on them WHY??? It is a RECOMMENDATION letter!!! I have plenty of weaknesses, but nothing different from most human beings. That is what the interview is for.
Personally, if a recommender told me to write my own letter I wouldnt know where to start and probably would not do it. But, I guess sonce it isnt uncommon, just highlight your good points, including your character, and back them up with a few examples. If a school contacts your professor to verify, which isnt very likely, they arent going to grill him anyway. They will just ask if he sent the letter, and maybe ask if he has anything to add. He may edit it before he sends it anyway.
Also, if a school is able to contact my D.O. I would like to know! I havent been able to get a hold of him, and I know he was supposed to leave the country for an extended period of time right around now. Hopefully he put his email address in the letter.