He wouldn't have a blaster, Obi-Wan wouldn't have managed to kill Grievous and would have died instead, Grievous would go into hiding, Vader would never lose his Force potential thus leading to him becoming miles more powerful than Palpatine, effectively overthrowing his master and becoming the unrivaled Eternal Emperor- of course, until Luke and Leia were powerful enough to bring him down. The cycle would repeat itself- the Skywalker family would become the leaders of the galaxy for a long long time. Until, of course, one of them never gives birth.
__________________ "Once more, the Sith will rule the galaxy! And... we shall have... peace."