I'm not sure ToME has a niche, but it certainly takes all the right hints from other roguelikes. Has an overworld map with tons of extra dungeons, large build variety, stuff like auto explore and convenient loot management (no moving stashes around or fiddling with weight or shop prices). It has more or less limited resource generation (there's an infinite dungeon and adventurer parties, but both are deathtraps in the long run in my experience, they're more like a last resort if you need more loot/exp and no place to go). It also doesn't have the kind of potion/scroll spam invincibility most other roguelikes let you have. There aren't really any consumables, instead you get things that mimic the effect of potions and scrolls(healing, regen, mana restore, curing stats, haste, etc.) But you only get so many slots, and they all have cooldowns, so you have to choose whether you want to be able to restore your mana or teleport or cure confusion in a pinch- you won't have access to all three (unless you give up regeneration or specialized towards having a lot of slots, and even then, there's always more things to want, you can never have enough.)
It also has unlockable classes, which drives some people crazy. Though tbh I think it's a good idea, the number of options available would be overwhelming for a new player, even if they have lots of experience with other roguelikes.