1. If the Farmer's power level was still 5, the scouter numbers of the fighters at the beginning of DBZ would be at least in the hundreds of thousands.
2. Gohan wouldn't have been able to bounce back the spirit bomb against Vegeta. He would have been hit by it and died. Or, if he were needed alive for the plot, he would have been injured, and then he would have transformed into an ape all the same.
3. Freeza's power levels would be a lot lower, or not everybody and their mommas would be a match for him in his first form.
4. The story Imperfect Cell told Piccolo about how he went back in time would be clearer.
5. When Tenshinhan saw SSJ Goku fighting Nineteen, he would have said something like, "Wow, he's stronger than 3 years ago."
6. Cell would have vomited Eighteen through his tail.
7. Cell's core wouldn't be in his head, since that had already been blown up by Goku once.
8. Gohan would have transformed only into SSJ1 at the Budokai.
9. Gotenks would have started out as SSJ against Super Buu.
10. Against Kid Buu, they would have done the reasonable thing and transported Gohan there to one-shot him. Or, if the author wanted Goku/Kid Buu to be stronger than Gohan/Super Buu, he would have explained it somehow. E.g. when Vegetto was defused, his power was shared between Goku and Vegeta.