One thing that drove me crazy about some Motorola and Samsung phones with 16+GB of internal storage, is that the storage was partitioned off and the usable space was mounted as if it was an sd card. This really wrecked havoc on apps that are supposed to save data to the SD card, because they would think the internal storage was the SD card and save to there. Not only was this annoying for some backup-based apps, but games which need to download anywhere from 500MB to 3GB of resources would install to the (usually 10GB or less after the rest is partitioned to the system) internal storage, thinking they were installing to the sd card. Basically, the sd card was only usable for photos, videos, and music... and file managers. The phone might as well not have an SD slot.
So far every HTC phone I have had hasen't been this way, the EVO 3D I currently has mounts the internal storage as actual internal storage, so apps correctly download these large amounts of data to the SD card. I didn't get a One because the lack of a SD slot was a dealbreaker for me, but now I am extremely interested in the M8 (Just a shame it isn't water proof like the S5 or Z2, oh well).
I am curious about how it treats it's internal storage however, since it starts out at 16GB(although it appears the Sprint version will be 32GB). Does it actually mount it as internal storage, so apps who want to write to the SD card would be writing to the real SD card itself, or does it work like the Samsung and Motorola phones where it partitions off part of it's internal storage as if it was external, making apps think that it's internal storage is the SD card?
I really hope it doesn't mount it's internal memory as if it was the SD card.