LabPlot2 is a complete rewrite of LabPlot1 and lacks in the first release a lot of features available in the predecessor. On the other hand, the GUI and the usability is more superior as compared to LabPlot1 and there are several new features that were not available in LabPlot1.
For a brief history about the beginning of LabPlot2 read the interview on dot.kde.org. See also a recent blog that demonstrate a couple of LabPlot2’s features.