Two features that are on excel but not excel online
1. Custom sort isn't an option online -- To sort by one column and then a second column and then a third column; to sort by a custom list that you create -- none of these are options online but are very necessary!
2. Currently on excel you can have multiple colors or formatting within one cell. It's a way to color code items. On excel online its either all red or all black, all bold or all plain.