Since I had a 29 weeker last time (he's perfectly fine with no delays, and is actually BIG for his actual age now!), I play the "baby is viable now" game too! Technically, 23 weeks is about the earliest a baby is viable, but there's still very small chance of survival at that point. I think the chance of survival is 50% at 24 weeks? There's a list of percentages somewhere on the net, and basically, each week baby stays in, it's better off, and a week can make a huge difference! There were several 25 weekers born this year on MDC, and they're doing pretty well.
My DS was born at 29w4d, but he was the size of a 31-32 weeker and did really really well - just 4 weeks in the NICU, and came home with no problems except some mild reflux that he outgrew by 6 months. But every baby is individual, and the complications they have while in the NICU can affect things. The only thing DS had going on was some mild apnea, which is normal in preemies until their nervous systems mature. And he had to learn to take oral feedings, and hold his own temp. He was breathing 100% on his own the day after birth (thank you steroids and surfectant), and within a few days, he was getting tube feedings instead of IV. So the only tubes/wires he had on him during most of his stay was the feeding tube going down his mouth/throat into his stomach, and the monitor wires that attach to chest and feet (not sticking in baby). As I said though, he did *very* well, and not all babies his age (or even older) do that well in the NICU.
Size can affect things too, including long term issues. I believe 3.5 lbs is the cutoff for the big risk of ROP (Retinopathy of Prematurity - an eye problem)? The smaller the baby, the more complications they're at risk for. So if your baby is big for gestational age, that *can* be helpful (although it doesn't always help everything).
As far as long term issues go... I've read that after 28 weeks, the chances of having a normal life with no long term developmental issues is pretty good. And IRL, I actually know several 28 weekers who have no problems whatsoever (most are high school/college age - they all came out of the woodwork when I was hospitalized for PPROM!). And the technology is better now than it was back then.
My big goals this pregnancy have been... 22 weeks without PPROM (lungs develop 18-22 weeks, and need amniotic fluid for that), 24 weeks (what I consider viability), 29 weeks (when I PPROM'd last time), 36 weeks (when my midwife can legally attend my birth out-of-hospital - what I *wanted* to do last time
I'm 25 weeks now, and if my baby came today, I think he would probably survive, especially since he is measuring big like his brother did. But I'd feel ALOT more comfortable if he waited until the 30s, preferably at least mid-30s, so the NICU stay can be shorter (or possibly even avoid one and have a take-home baby!). But hopefully, I'll avoid the hospital altogether. Maybe I'll be posting here at 43 weeks, saying "Ok, you can come now!".