Well, I've played through most of the first level so far, here are some of my thoughts.
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The entrance - nice entrance. My first thought was oh no, please don't tell me that I'm going to have to clear out the whole lot of tar, but then I realised of course that it was quite easy to move around.
I headed south but seeing the exit and blue doors decided to go north first.
1 north - this was quite cute.
2 north - I'm not a fan of walls-under-trapdoors rooms. I didn't even need to hit any of the orbs because you can get to the mother in time.
3 north - nothing hard here. (not saying its a bad thing, not much I can say)
4 north - I see the blue door and start to worry - if the south rooms are required, will I be having problems with that blue door not dropping (I haven't found out yet).
5 north - took me a minute to actually realise that it was actually writing, I was too busy watching the queens

Nice in the fact that its not as hack and slash as you might first think. Checkpoint somewhere?
6 north - pretty easy room, I thought at first you might need the slayer to help with the tar but you don't.
7 north - again, a checkpoint would be nice somewhere.
8 north looks quite cool. I like the use of coloured doors in several of the rooms. (Again, a checkpoint please?)
9 north was a typical mimic movement room, though I don't seem to remember still roaches being used before, interestingly (eyes, definitely, but not roaches + invisibility).
Currently stuck on 10 north - I can't seem to prevent one of the Seeps from getting stuck.. a checkpoint is definitely needed here, its a bit annoying having to kill all those top left monsters each time I want to try a new attack on the seep. Of course, the monsters may have some involvement that I haven't realised yet, but a checkpoint would be nice anyway..
(I use the mimic to hit the orb while standing up by the seep, but still can't avoid it..)
Overall, pretty good so far, a few more checkpoints needed. And I agree about the linear thing.