My father retired at 55, and my goal is to retire at 54 (just shy of 2 years from now) because I'm competitive like that. Virtual empty nester (one 18 year old still at home, but works full time and barely ever home, probably will move out within a year or two), no mortgage, been piling cash into my brokerage account for years now. I'll be drawing SS benefits the day I'm eligible, where my wife will wait till the last possible moment (she is about 5 years younger than me).
Obviously, like others have alluded to, the big issue is private health care for myself and my wife until Medicare hits.
But, there are a few other issues that I need to tackle in my mind first. First, what to do with ourselves. Sure, I have a hefty reading list, and movies I'll watch and travel will be pretty heavy at least 55 - 65/70. But I mean, that still leaves quite a bit of free time. I love golfing, but I think 1x per week like in a league would suffice. My wife is already looking into substitute teaching gigs as she enjoys teaching (part time, she is not a teacher), but I need a few more things to fill my time. Hiking, gardening, gambling, may join a foodie group or cooking group or something to stay active.
The other issue which I think is going to be harder for me to get a grip on is spending money. I'm real good at making and saving money. I'm relatively good at investing and budgeting. But spending over budget and drawing down from savings is going to give me real anxiety. I will get a good test of that this coming year as I have a ton more expenses than earnings (Daughters wedding, 3 additional trips, few graduations out of state, etc...) so I'm already trying to figure out where to pay for all of this from, but we're a still dual income family. When there is no income other than interest/dividends and draw downs, well that's when the anxiety will set in

Will try to revisit this thread periodically as well.