If elves had gone on the quest instead of Merry and Pippin

hmm, now there's a thought... no idea about whether Frodo and Sam would have been able to go to Mordor alone (although i suspect they still would have got away). The main change i can think off, is that without merry and Pippin becoming Rider of Rohan/Soldier of Gondor, the shire may not have been saved at the end. Since it is those two who organise the defences....