As I've said before, I don't have that much experience with Arriva as I lived deep in GNE territory for most of my life, it's only in the last 5 or so years that I've had to deal with them.
As much as I love complaining about them (I think we all do!), they don't do /that/ much bad, sure they have their bad points, but they also have some good ones as well! 
Bare in mind that my experience is limited to services that operate around County Durham

Some of my complaints about Arriva:

I've yet to see an Arriva driver that looks like they're enjoying their job, they always look miserable!
Many times I've had drivers that barely speaks English or doesn't know places along the route (usually a mix of both)

MAX and Sapphire are nothing more than a name at this point. Sapphire was meant to be their 'premium' service but that didn't last.
They have have so many different standard liveries floating about you'd think they were different operators!


I have no issue with getting older vehicles from other divisions, but when you 'upgrade' a route with 11 year old buses, that's when I start having issues
Their vehicles are pretty much always dirty inside
They never seem to repair damage to seats. There are seats on fairly new buses that are all ripped, and if you're lucky they're even taped up!

I would bring up timekeeping, but I know that's not always down to them. Although if the timetables were planned properly they could take this into account, especially if they're a regular occurrence.