Why is it a terrible wonder?
Since no one answered you really, here's why.
Your early game goals are to expand to claim the best city spots, build infrastructure in your expansions and capital and to get the national college in a reasonable timeframe (usually <T100.)
The GL plan forgoes ALL of that in order for you to build an early 1 city NC. This is inferior for many reasons. First of all, the science bonus you get scales with your raw science. 50% extra science on 16 science isn't exactly game breaking. Yeah, you run thru techs fast, but what does that get you? Your build queue is still backed up and is limited by the production you put out. You can't build temples before you build shrines, for example.
The other main reason is that your first priority in the game should be to get your new cities settled. It's as simple as claiming your spots before the AI claims them. In addition, settling your cities 30 turns earlier means they all have 30 turns to grow, which will undoubtedly build you a better empire than waiting until T50-60 to get your expansions out.
It's not so much that the GL is a bad wonder, it's that it's cost is too high, even if you do manage to build it. One free tech and a library which should take no more than 5ish turns in your capital when you're done building settlers is just not worth losing out on all the city space, infrastructure and growth you're giving up.
It's a noob trap, that's all. It seems like a good wonder. It's just bad compared to what you want to do with it. If it came at Education, it would be an amazing wonder, but it doesn't. It comes when you really want to be doing other things with your empire.