I must admit I don't use it a lot in tablet mode compared to the desktop but there are little things that annoy which I've mentioned elsewhere on the forum...
- Edge is frustrating to use due to no swiping to go forwards/backwards and the address bar being active
- I haven't found a quick way to get rid of the Taskbar? Again, you're losing real estate of the screen
- I used to love leaving the Tiles on the Start Screen to auto-update. This isn't possible any more
- When docked, swiping in from the edge now opens options on the main monitor, not the tablet one
- Intermittently flicks between desktop mode and tablet mode, obviously a bug but was fine on Windows 8
Had to edit to add one more which is driving me
crazy. The fan runs almost all the time in Windows 10, making the Surface very hot to hold (not to mention noisy when laying in bed) within Windows 8 I hardly ever heard it... really just when running HD videos or converting media. This is the biggest disappointment out of everything for me