I might not be in the majority, but I actually like the new system font Helvetica Neue. I am a huge fan of it and on a Retina Display it makes so much sense. Understandably those who do not have the luxury of a Retina Display, I can understand the frustration, but having said that, I think the whole system font change thing has been blown out of proportion.
Even though I like Helvetica Neue, I really dig the new San Francisco font, it is a pretty nice alternative that seems to work pretty well for those who are using a non-Retina Display. If you want to bring back Lucida Grande, this Github repository has a handy script that will do that for you (some work colleagues of mine, designers mainly did it to bring back the old font): https://github.com/schreiberstein/lucidagrandeyosemite
Even though I like Helvetica Neue, I really dig the new San Francisco font, it is a pretty nice alternative that seems to work pretty well for those who are using a non-Retina Display. If you want to bring back Lucida Grande, this Github repository has a handy script that will do that for you (some work colleagues of mine, designers mainly did it to bring back the old font): https://github.com/schreiberstein/lucidagrandeyosemite