The first Green Lantern film came out in 2011 and the reboot is planned for 2020 9 years apart, FF was 10 years (first film) and 8 years (for the sequel) apart from the current release so the time frames are basically the same. Ratings on the site Rotten Tomatoes are very close too GL got a 26% on the Tomato Meter and 46% audience score, FF got 27% and 46% respectively (the sequel did slightly better in both). Also per Box Office Mojo GL made $53 million opening weekend and $116 million overall, FF did $56 million opening weekend and $155 million overall (the sequel came in with $58 million and $131 million respectively).
There were very similar complaints on both films, with costumes (poor CGI for GL, rubber suit for the Thing), actors/acting, and the villains, especially in GL and the FF sequel, that many looked at as just a big cloud (Parallax/Galactus) rather then a noticeable, believable villain. Like the FF reboot Green Lantern will need to have differences to set itself apart.
There was good in the first film but to try to make it better, and different, they may lose some of the good stuff from the first film like Mark Strong’s performance as Sinestro. In the case of the first and current FF both films are origin stories the 2005 film’s origin was fine but in the new film they were given a new origin and it just doesn’t work well in the film. For GL they need to stick close to the first films origin, maybe just tweaking it slight, and not go with a very different origin. Or they may need to jump forward like what seems to be the case with Batman and skip the origin story at least for now.
DC and Warner Bros need to be careful in how they go about the whole thing, something that may help GL is that it’s included in the DC Cinematic Universe and not a solo franchise this time around, also there is the possibility (and most likely) that GL will be introduced before the solo film hits in the Justice League film(s). They have already done this in a small sense by not announcing who is playing GL despite a number of actors pushing for the role, although it could backfire at the same time by being too cautious. Another thing that may help GL is word that there will be multiple Green Lanterns for Earth not just one this could avoid fans focusing on one actors performance as the title charater, although it didn’t really help FF at all.
Whether FF issues are just related to studio/director/story issues or something else DC and Warner Bros definitely need to keep it in the back of there minds going into the reboot, DC fans really want them to one up Marvel and so far they haven't but it is still very early and we will have to wait to see what they can do.