I've had the side panel off my PC for a while, and it runs pretty cool. It feels like too obvious a solution though... everyone would just do that if it worked. So I'm wondering if removing a panel is really a good way to keep your PC cool or is there some reason I shouldn't? — Mark
It's a "do as we say, not as we do" sort of situation. That photo above of the LPC shows the side panel off, but that was just to show off its guts. In practice, you probably aren't going to solve your cooling issues by removing the side panel of your PC.
The principle of air cooling, as described well in this ExtremeTech guide, is to create either positive or negative air pressure within the case. Both have pros and cons, and there's tons of disagreement over optimal fan placement, but the point is: bring cool air in, heat it up, and get it out. When you remove the side panel, you disrupt the intended airflow of the case—whatever it may be—and also invite more dust to settle on your components. So it's not recommended.
If your system runs significantly cooler with the side panel off—except in some very specific situations I might not have anticipated—your case and its fans are either designed poorly or working poorly. I tested my PC at home with the case on and off, and the difference was negligible (everything was well within safe ranges with either configuration). I'm also a terrible example, because I'm horribly lazy and often leave the panel off when I'm swapping lots of parts. Remember, not as we do...
So, while removing the case panel won't be catastrophic, it counteracts the intended cooling solution of your case, and even if it works without the panel, there's the dust problem. If you really want that open air look, then yes, you can safely run a caseless computer—our friends at Max PC pointed out a couple years ago that natural convection can keep a test bench PC within safe temperatures, assuming the room is kept reasonably cool. There's still the dust, though, and I'm very anti-dust.
If you have a case, you're best off leaving the panel on and letting it work as intended. And if you're having serious overheating problems, then look for the real issue. It could be a problem with your CPU cooler, your PC may need a cleaning, the fans may not be operating properly or are obstructed (cable management is important!), or your case may just have terrible airflow (sorry). Any of these cases would be a good choice for your next build, and until we have an air cooling guide of your own, I'll refer you back to ExtremeTech for more specific tips.