What makes a light such as Surefire more suitable than others to use on a gun?

I'm not talking cheap Lowes or walmart lights.

I've heard people mention that decent lights such as the Fenix PD series and Olympia Rg series as not truly weapon lights. They're good for EDC, but not mounted to a weapon.

I imagine its the ability to absorb recoil better, but with all of them using impact/shock resistant led's what other parts tend to fail sooner due to excessive or long term recoil?

Well, I can give you one example of a light fail.  I have a few Streamlight PolyTac lights.   Great light, but no good on a shotgun.  The button is on the end, and every time you fire, it turns on or off.   Works fine on an AR.  I would say that the answer to your question is that they are designed and tested to avoid things like that, that aren't factors for a non-weapon mounted light.

Since SureFire is well known for weapons lights I'll use them as an example. The M600 incandescent (non LED) weapons light body was machined with a wall, used a plastic grommet, a heavy rear spring on the tail cap, heavy front spring on the 6P lamp module, and a shock absorbing head which also acted as a buffer to help protect the filament from recoil. Incandescent lamps has it pros of being able to filter white light for IR only, better ability to cut through fog and smoke. The biggest con was the output was limited due to the batteries and a shorter run time. The biggest pros of LEDs was the higher output, battery run time and no filament that broke under recoil. The biggest con was in the beginning most Chinese lights had a blue hue/tint to them. The reason why I don't run Chinese lights as a weapons light is that the build quality, materials and QA is not  the same as a SureFire IMHO. Now I know there will be a lot of posters that will dispute my statement tp the end of time. What I've seen the materials used and the soldering of the springs to the board is done with spot solder which can break off due to recoil. If you resolder these connection and use quality o-rings you can have a reliable light although I still would not use it for EDC/HD/Duty it will make for a decent work light. Since Surefire is competing with the Chinese and other LED lights out there they do make a lower price G2X light that some have used a weapons light even though SureFire doesn't recommend it. The first ones had a very low profile forward spring which was doomed to fail under recoil, so SureFire updated it with a bigger one although not as robust as the ones used on the 6P lamps. While the G2X may work on a 5.56 light use carbine I wouldn't use it for HD/Duty or on anything with a higher recoil impulse. If you wanted to upgrade a most Surefires to a better module that had longer run time and built even better for recoil etc look at a Malkoff drop in. Since SureFire is building their lights with the almost indestructible LEDs with good springs they are not machining their bodies or using any of their old isolating techniques used on their older weapons lights any longer.

Thanks guys.