The hair everywhere, all the time. I hoover every other day, I wipe all the skirting boards every few days, I clean the walls when I notice marks - there's still fucking hair and dust everywhere from them. I mopped the floors today and they immediately came in and did a wet paw tap dance all over them, so that was a waste of effort.
Dog smell - we wash our dogs semi-regularly, they're not allowed on the furniture, I keep the house pretty clean - I can still smell them.
They follow you constantly, are always underfoot, have no regard for your needs or desire for space, and you have to walk then every day, regardless of weather, tiredness, being busy etc.
The commitment - can't go anywhere for too long or do anything spontaneous. Going away, even for a day, entails additional expense for a dog walker. Going away on a holiday means hoping you have nice friends who will have them (and your dogs are civilised enough that they won't be too much of an imposition), or paying a boarding kennels a massive amount of money.
Picking up dog poo, your garden basically becoming a glorified dog toilet unless you're willing to get out there on a turd hunt in the pissing rain every day.
My dog can open doors, so constant scratches and paw marks on doors and walls when she's decided she'd like to be somewhere different/join me when I'm trying to have a solo wee.
Their constant propensity for eating new and disgusting things, followed by yakking them up or shitting them out at high velocity all over everything that is dear to you.
Puppies are small, cute wankers whose entire purpose in life is to see if they can break you. My dogs now are lovely, well-trained, pleasant household members, but one of them genuinely almost tipped me over the edge as a puppy.
Insurance becomes massive if they live a normal lifespan and get old. Vet bills are astonishingly high. If they get ill it will always be when you need an out of hours vet and the consultation fee alone is over £300. The insurance payout limit is crazily low if you have a pet with any kind of complicated or chronic health condition.
Despite all of this, I love mine huge amounts, but this leads to its own problems - I will be heartbroken when they die.