Mudguards, mudguards, mudguards!!!
+1
It used to be that, apart from racing bikes, mudguards were part of any sensibly designed bike. Problem is, so many road bikes are being built without enough clearance and attachment points for mudguards. This fact often is the basis for
my most popular rant--that racing too often influences the design of bicycles in general.
Happily, there is a trend towards utilitarian cycling (which is what most of us do most often) and bikes appropriate for the job.
As I've said here before, I was surprised the last time I was in the UK (2007) to see so few classic British bikes, the kind I grew up around. Most people seemed to be riding department store mountain bikes. That has been the case in Canada, as well, so I'm glad to see the aforementioned trend. As more people demand utility over pretence, we'll see better design.
As for riding in the rain, I'd also add a good mudflap, at least on the front. You'll be amazed how much muck a commodious flap keeps off your feet (which will also benefit from booties). Tyres should also be considered. No knobbies needed; the more rubber on the road the better. Watch out for grease (the first rain after a dry spell is dangerous), manhole covers and road markings. Keep the rubber side down.
Happy Trails!