Many people, not all, have a problem with lines ( more pronounced than usual ) around every block. No matter what they do with the ingame video settings, nothing seems to help, despite having a fast system. This doesnt happen with everybodys systems, but for those that get it, it makes the game almost unplayable.
Well, there is an easy fix:
At the moment, Minecraft doesnt like anti-aliasing. And you cannot change it ingame. So all you need to do, is goto your graphics card settings, and FORCE it to turn OFF anti aliasing, either for minecraft, or totally and then change it back after playing.
Hope that helps some people.

Well Jayjay, i use an ati card, so i have a catalyst driver on the toolbar, to tinker with settings. Nvidia cards will have a similar thing, look for nvidia control panel, or something like that.