It's not just the look of some of these new sites I don't like - sometimes the functionality is baffling. Some time ago I purchased an app called Todoist for my iPhone, it's one of those fully featured task management systems and there's also a desktop site you can use in conjunction with it. To this day I'm at a loss as to how to set up tasks and have them send me a text notification. The GUI is such that you never really know what's going on, the navigation between functions doesn't make any sense to me and indeed the whole thing is totally confusing. What makes it worse is that the desktop site seems to work differently to the phone app in many ways, and it's almost impossible to get worthwhile answers from their documentation because the trend these days seems to be "hey we don't need to provide instructions because our app is so simple (to everyone who worked on it, anyway)"
And the flip side of the "sites that are too big and wasteful" problem is older sites and apps that seem to have been designed for the old 800x600 resolutions and never updated. I've recently been trialing some web apps to automate my business and I tested one that is actually very powerful in functionality, but it looks like it was designed in the days of DOS and everything is scrunched up in one corner of the browser. After an evening of trying to get to grips with it, I realized that I was squinting and had a headache. I put this problem to the developer over the phone and he didn't see the problem at all. I actually suspect that he's still using an old CRT monitor.....
Another pet hate of mine is developers who constantly rearrange their GUI's and settings so that you never know where to find anything. Google are the absolute worst at this, as anyone in IT who has to deal with Google Apps will tell you. They never stop changing stuff and just when you've gotten used to one layout, boom, they've changed it again. Settings you use every day are now in different tabs etc. When you Google for the help documentation, you find that all of the results are outdated and refer to earlier iterations of the software. I find this a lot with YouTube, they are always changing the settings so Googling "How do I......" usually gets you nowhere.