Fellow 12/12 cycling commuter here. Based on my experience I'd say, in order of priority:
- Internal hub gear.
Rear derailleurs are a pain in the backside IMO. Need regular adjusting and cleaning. I really miss the IHG on my previous bike.
As well as being low maintenance and reliable, they also have the advantage of being able to change gear without having to engage the crank (useful for example to change to a low gear to move off after a sudden stop).
The only disadvantage is slightly more weight, which on a commuter bike is irrelevant.
The Shimano Nexus hubs (7 or 8 speed) are good value for money - require a compatible shifter, usually a twist-type.
The more coverage the merrier. When you have an IHG you can have a full chain guard.
Mud Flaps/"Fenders". Reduces the amount of muddy water getting on the bike, you, and the poor sod riding behind you. Actually a requirement to be road legal in my part of the world. Add little rubber faps at the end.
Good Tyres and reasonable quality innertubes.
Puncture resistant tires (e.g. Marathon Plus from Schwalbe) are a worthwhile investment. I still have a quick-repair canister in the bag just in case.
Going tubeless might be an option but you'll need appropriate rims and I thinks its a bit faff setting it up. YMMV.
I've found roller brakes (e.g. Shimano Nexus series) to be fairly low maintenance compared to other types - occasionally (couple times a year, depending on your riding style and terrain) need to add lubricant/fat. They don't provide the best braking performance - if you have a lot of extreme downhill you need to pump them. Not usually relevant for commuting.
Hydraulic disc brakes, despite my initial scepticism, are also surprisingly low maintenance as well as providing very good braking. Self-adjusting. New pads once a year (again depending on riding style, etc.) which is easy to do. They will need bleeding at some point which is a bit fiddly but that's less than once a year in my experience. Just make sure you don't engage the brake lever when the wheel is removed.
6 Belt drive instead of Chain.
Less maintenance than a chain, though you need the IGH in point 1. Does have the disadvantage that if it does break it's not as trivial to replace.
Have a look at mid-level European city and touring bikes - they'll have most of the features mentioned above.
An aluminium frame is probably the best bet.
If you live somewhere with proper winters and the roads are salted, you'll need to still clean it off regularly otherwise risk of rust and corrosion. Consider using bike polish before winter starts.
I have the impression that having some suspension (e.g. hydraulic fork, sit-post with at least a spring) eases stress on frame and components (with drawback of potential additional maintenance). Depends are what the road conditions are like on your route.