As you can see by this thread, OP you're definitely not alone.
I think it has to do with a lot of things, least of which means all of us have bratty spoiled rotten kids. Kids have their phases and add to that heat, humidity, possibly overstimulated and/or tired... it's not a good mixture with kids.
Our first few trips weren't bad at all- they were with DH. Difference then being the kids were pretty much well-behaved the entire time but the comment one person made about the DH having temper tantrums about the kids' temper tantrums pretty much explains how that went. Not so much the 2nd trip that was longer (12 nights) because we had a lot more down time for DEAR HUBBY! LOL It was like he expected them to never get the least big whiney or gripey or tired... but really it was just about he doesn't really get into Disney. That's why the 2nd trip with pretty much every other day spent at the resort, pool and/or DTD or Disneyquest, etc. was better for him.
However, then on the 2 trips I took this summer with the kids... the first one with all 3 was pretty stressful. They weren't misbehaving the ENTIRE time but the youngest one was whiney enough to make up for that. LOL We were all pretty grouchy- it's our first summer trip and even though we're from Texas the heat and humidity even had ME feeling pretty miserable and therefore grumpy at times! Add to that the fact we couldn't do anything SHE couldn't do (and she's not that tall) - that caused a lot of disappointment and griping as well. Well, we decided (last minute- partially because the youngest was ill at the time) I'd go with just the older two the next trip and even though it was a much shorter (half as long) trip it went great. We could all ride the "big" rides and there was only 2 of them- easier on mom (me!) to keep the fighting to a minimum (and less stressful keeping an eye on them, I didn't have to push a stroller, etc). There was only ONE time we had a meltdown waiting in line and I threatened to leave the line, they didn't believe me and the arguing kept up -so we left the line. My kids have a tendency to just want to argue the most ridiculous stuff because they both think they are right. They don't fight over things like who gets to be by mom or who is first in line- they argue about things like well- hard to explain. Just pretty much anything they see/hear/think they know, they will argue about it because they both think they are right. So it's all verbal- and they don't yell/etc. but it's like
constant nuh-uh, you're wrong, I'm right stuff- and it gets annoying after awhile. We left the line and did a little "time out" while we sat on a bench -- no more problems after that! They knew mom meant business! I think they thought there wasn't a way in the world that mom (aka Disney freak LOL) would pass up a ride or get back out of a long line/etc. if they misbehaved- they found out different and behaved better after that one incident.
UNTIL the last day.. OMG it was awful the last day! Thankfully it wasn't both of them, it was just my son. I don't know what in the world happened but that boy woke up on the WRONG side of the bed or something. From the minute he got up he was whining and arguing and being pissy about everything. (he's 7yrs old btw). We were leaving late that day so we had to check out and also check in with the airline and then headed to TL for about 5hrs to then return to the resort, eat and get ready for DME to pick us up. All day long- gripe, whine, piss and moan about everything. It was like he wasn't even the same child anymore! I still don't know what caused it- he got plenty of sleep and took his medicine that morning (he's ADHD/possibly bipolar) but apparently it did NOT work that day.
He was fine once we got on the airplane. Go figure. Apparently that 5 night trip was a bit much for him even though we'd done 10 nights back in June?

It was just a bad day for him I guess. I can't even IMAGINE what that day would have been like had I also had our youngest daughter with us on that trip. Lord have mercy it would have been bad. LOL
We all have our days I guess.
I wouldn't give up going at all- but if you go again with the kids there is LOTS of great advice given on this thread to hopefully nip bad behavior and fighting/etc. in the bud before it gets to the point that the trip is ruined and you're left in tears wondering why you even bothered going. Or you COULD try to go on either a solo trip, with someone else or with DH without the kids (if that's possible) and maybe that would be a rude awakening for them. My kids KNOW that I have gone on solo trips before and I'm not afraid to do it again! And now they know whiney kid gets left behind next time (since I left youngest behind on the last trip) because I ain't having that at Disney World! Nuhuh... don't mess with my Disney. LOL