what GPAs correspond to what kinds of job prospects in which industries
CLS doesn't really disclose GPAs, they go by honors. From what I can tell, there's Kent scholar at a 3.8 (top ~5%) and Stone scholar at around 3.3/3.4 (top ~30%). Only the very few selective big law firms with have a hard cut-off for certain GPAs, but if you want Wachtell/Cravath/S&C (or want to break into a hard market like DC), you need very strong grades. For the most part though, I've seen a range of people get very successful offers from all around the grade spectrum. That being said, I would not want to go into EIP (Columbia's on campus interview program) with straight Bs.
the relative weight of ECs/majors/other softs to prospective employers
Grades are probably the single biggest factor when it comes to law firms, though there are several nice softs to have. Really good work experience (IBanking/Consulting, or even paralegaling at a top firm) will give you much more to talk about in a screener interview that that lunch you planned for your Columbia Australian Law Club. Related here would also be military experience or other degrees (especially in a hard science for patent lawyers). The only soft employers seem to care about is Law Review (yay status symbol?), though I'm sure it's a good thing to pad your resume with a club or two related to what you want to do.
which classes would be important to take vs which classes were a waste of my short time at school
Doesn't really matter for your first year, everything is planned for you. Get here, study hard, get kick ass grades, get a summer offer (happens in August/September after your first year) and then you can figure out which classes you want to fill your last two years with.