What's Fit to Fly
Ok, the difference between the "fit to fly" standards is in FAR (14 CFR) 61.23, specifically 61.23(c). Eventually, the rule gives you the following criteria for SP operations if the pilot is using a DL instead of a medical:
(2) A person using a current and valid U.S. driver?s license to meet the requirements of this paragraph must?
(i) Comply with each restriction and limitation imposed by that person?s U.S. driver?s license and any judicial or administrative order applying to the operation of a motor vehicle;
(ii) Have been found eligible for the issuance of at least a third-class airman medical certificate at the time of his or her most recent application (if the person has applied for a medical certificate);
(iii) Not have had his or her most recently issued medical certificate (if the person has held a medical certificate) suspended or revoked or most recent Authorization for a Special Issuance of a Medical Certificate withdrawn; and
(iv) Not know or have reason to know of any medical condition that would make that person unable to operate a light-sport aircraft in a safe manner.
Ok, of these, the one that is important is (iv). (i) is fine - comply with your DL (e.g., wear your glasses). (ii) and (iii) are easy - if you applied for a medical, you had to have been found eligible and not have had your medical suspended or revoked.
But what to make of (iv)? It puts almost all the responsibility on the PIC to certify that (s)he is fit to complete that flight. So how do you judge whether you're fit to fly or not? Most likely, that will only be addressed after an accident or perhaps during a ramp check. If you have an accident that is pilot error and is related to a medical condition, then you have apparently failed the test of 61.23(c)(2)(iv).
However, this is very different from the 3rd class medical requirements and it is also different from just meeting the medical requirements of a DL. Suppose you have some sort of lung disease from smoking. You have problems breathing at higher elevations (e.g., DEN) but live at sea level. You can get a DL but might fail a medical. Are you fit to fly? Probably not.
Now suppose you have, oh, let's say chronic migraine headaches. You think you're not going to get your medical renewed since there is now a medication that you have found can prevent them, but it is not allowed by the FAA, so you let your medical lapse and go SP. Can you fly as a SP? Are you able to safely complete the flight? Only you can make that call.
TODR