There are plants that will quit/slow down flowering if not deadheaded--once they're able to make seeds, they have no more reason to flower. Geraniums, as a pp mentioned. Petunias, in my opinion, get leggy and ugly if they're not pinched/deadheaded.
Other plants, like calibrachoa, are self cleaning and don't need deadheading at all.
Another lazy gardener here. I deadhead geraniums. I will snip off faded blooms for some plants like zinnias, but I like to allow them to go to seed so that I have some volunteers. Snapdragons self seed pretty well for me here (zone 7b). I also tend to buy plants that are self cleaning (angelonia, vinca)