Hardware.
Samsung has a history of making good/great hardware (fast CPUs, high quality displays ect) that include features people want (SD, removable battery). There was a trend around the time the GS2 came out where Google was trying to push Android vendors towards the iPhone model (no SD expansion, and embedded batteries). Samsung resisted this trend while still providing great specs. And a lot of people liked them for it. That was before the touchwiz backlash started.
The M8 has an awful camera. It is only 4MP and it's quality is worse than the much older and much cheaper Nexus 5.
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The did finally bring back SD support...but it is probably too little too late. There is really no reason to buy an M8 at all except for the case and the stereo speakers. The Galaxy S5 has a better camera, SD, Removable battery,
the best display of any phone on the market right now, and is waterproof.
I owned an HTC Rezound...I loved the hardware, but I grew to despise Sense (HTC's custom UI, similar to Touchwiz). I will never own another Android phone that does not use Vanilla Android. I've since installed a Vanilla Android ROM onto my Rezound...it runs significantly better and has the latest version of Android
now.