I suppose this would be a "puzzle platformer" but there's only 1 or 2 truly tricky bits and with a bit of practice they aren't too bad either. For me the gameplay feels almost secondary to the very well done narrative that really drives the game forward. It's high quality narration and while it never gets overly detailed in terms of plot and such, as an adult (sort of) it's just genuinely nice to be told a story. You can certainly find some depth to the story too. It's very tempting to draw some comparisons to The Stanley Parable, but I'll resist because they're actually quite different games.
Having said all that, the gameplay in Thomas Was Alone can still stand up on it's own. As I mentioned earlier it's not too hard for the most part, but the pacing is good and the game makes sure to switch up it's mechanics just enough so it keeps things fresh but doesn't disregard natural learning progression either. The controls play fine, and the levels are all pretty well sized. Whilst it's not breaking too much new ground, it's very well polished and those familiar to the genre will probably enjoy themselves.
The graphics are pretty basic, but not in a derogatory way. They fit the style of the game well, and there's some nice effects along the way. I ran this game on a cheap laptop from 2008, and it was a little shaky at times but very much playable. I shouldn't imagine anybody will have issues with framerate or anything like that unless you're seriously behind in computing power.
Overall I'd recommend this to even the most casual fan of narrative. To me it seemed enjoyable in the same way a cup of tea and a slice of carrot cake is enjoyable on a horribly cold day. It's warming in all the right ways. It's not often I get carried away and leave a glowing review of anything, but Thomas Was Alone seemed almost tailored just for me.
The game does really good in terrifying and scaring the player. When you're facing monsters without the ability to kill them, only to hide from them, it can get very, very tense.
Also, they've done an excellent job with the atmosphere in the game. The background music, notes and story altogether make for a very creepy, yet interesting atmosphere. The story is good, and there is a clear goal to the game at all times.