Mendeleev's periodic table was helped more in creating modern periodic table as he arranged elements in the increasing order of atomic mass and moreover, he tried to arrange the elements on the basis of their reaction with hydrogen and oxygen because they are most reactive elements, react with most of other elements. In his period, 63 elements were known. So, this made quite easy for modern periodic table.
Here are some predictions of Mendeleev
Advantages of Mendeleev's periodic table
Elements were grouped on more fundamental basis.
Because of their placement in the periodic table, properties of several elements yet to be discovered like scandium, gallium and germanium could be predicted.
scandium predicted as Eka boron
gallium predicted as Eka aluminum
germanium predicted as Eka silicon
Gaps for 40 new elements were kept.
The discovery of new elements inert gases did not disturbed the arrangement and they found their place as a new group.