Let's start a list:
Opening a csv triggers a full recalc even if your calcs are on manual. That's absurd given that csv files by definition have no formulas
Come on Microsoft. 18 years without updating the scripting abilities. Can't we have a .net version of VBA?? And NO JAVASCRIPT.
The fx button still opens a tiny old non resizable windows with a broken search feature, which is impractical if you have hundreds of UDF.
High levels of nesting in the UI making it really slow to repeat certain actions (try drawing something). I still miss Excel 2003's customizable UI.
They could provide at least a function to pull end of day FX rates.
Integration with powerpoint pretty much non existant. A huge amount of users prepare decks based on a model in Excel. The process to link them is unstable / stone age.
We should be able to transform a sheet into an excel formula, so that a non programmer can apply a complex logic to lots of data without a script.
Xlsm files get corrupted regularly when excel is running low on memory.
Adopt apple's idea of a sheet being a canvas on which you can drag tables or charts instead of having this antique model of a single grid which becomes annoying as soon as you have two tables on the same tab.
I am sure I can pull more if popular demand!