I think in terms of mods the biggest disappointment for me was trying to find someone to dyno tune my car only to find out no one does it and having to get a mail order. Not really a big deal but disappointing. I also bought LT headers that were really poor fitment not sure if the issue was with the headers or my car to be honest.
Overall though not really a mod issue but the one thing that irks me more then any thing is how other people i know are able to push there stock applications much much further than we are. I would love to have added forced induction to my camaro but the total cost of doing it is just insane. so instead i just tinker and shoot for a 12 second car.