I haven't been to the park in 7 years, I had annual passes a few times when I was first married but now I have a kid.
What should I expect? how do others 6 year olds handle the park? too much stimulation, too little? frequent temper tantrums? Disney induced drug like haze? any input is appreciated.
We took our son for his first trip a month after his 6th birthday and he loved it so much we are heading back for our 7th trip since 2011!
Now, please read all of this with the knowledge that our son is autistic so not all of this will be applicable for your son.
Overall, he dealt really well with the park although we did have some touch and go times where things just got to be too much for him and he just could not cope with the overstimulation. We made sure that we went back to the hotel after lunch for a break and on that trip and the one after that he actually fell asleep and napped for an hour or so. He is 9 now and we still go back to the hotel for some quiet time to allow him to decompress a bit.
Snacks are a must, low blood sugar is a killer and no one gets crankier faster than my son when he is hungry!
He has issues with lines and people crowding him so we use the system that
Disneyland has in place to help kids like him but when we do go through the standby line (around 50-75% of the time, we are working on him leaning how to deal with the world instead of the other way around!), we allow him to bring his game system to play. His Nintendo DS with headphones gives him a way to shut out the outside stimulation a bit and keeps him from getting overwhelmed. I know that some people look at our little family and think that we are lazy parents for letting our son play with electronics while we are in Disneyland but I just ignore any looks.
Souvenirs...by far the BEST piece of advice came from some here. We buy him a new set of ears every trip but other than that he brings his own money for a souvenir, usually money given to him by grandma! But, what we do is whenever he sees something he wants to buy, we take a picture of it with my phone. Then, on the night before our last day in the park we look through the pictures and he gets to pick what he really wants to buy. I cannot say how much this trick has helped! Our first trip we ended up with a bunch of things that he REALLY wanted right that moment and after playing with it for a day he was not as interested anymore. This trick gives him a chance to think about what he really wants.
Make sure everyone stays hydrated. Even if he does not think he is thirsty, I keep an eye on his water intake. No one wants to get heat stroke on vacation!
When he says he is done, then we leave and go back to the hotel. There is nothing to be gained by pushing him.
We used to try and catch the absolutely earliest flight in to Anaheim and we would be in the park by lunchtime. This only resulted in a first day meltdown because in order to get there that early, we had to wake up at 4:00 am to get to the airport. It took us 6 trips but we finally learned our lesson and this time we are flying in the night before so that we can get up at a more normal time and hopefully avoid the first day disaster.
Lastly for now, I try and avoid saying "no" when we are in Disneyland. It's a few days out of the year, why not say yes as much as possible. Ice cream for breakfast? Sure, eat something healthy then you can absolutely have dessert at breakfast! Want to ride Star Tours 4 times in one day? Sure, as long as we can get fast passes or wait in the line, or course we can! Pretzels at 8:00 pm? Why not!
We have so many times that we say no, it is really nice to be able to say yes as much as we want!
!!)at DL 
and if you do we implement a time out. Yep I am the Mom you have seen sitting with a tantruming child 
while the rest of the family went on a ride! Luckily my kids are quick learners so this has only happened about 5 times in 3 trips!
