Warning: I'm not a professional designer, so do not take my notes too seriously.

Generally I like the whole spirit of it, but you follow too strictly the geometrical proportions - which does make it look somehow unbalanced. Visual balance is not an exact plain geometry.

C is too wide, while H is too narrow. K is quite close to H, so try to make the difference between them more obvious. M seems too wide, but maybe it's just me; same goes for U. One of the hardest ones to draw - S - looks nice, but again unproportionally narrow. Generally you get the idea, the width of some of the characters bothers the eye.

The height of lowercase t is too small - no ascender at all, and looks considerably thinner than the other characters - obviously this is just an optical illusion, but you need to compensate this. Here I should repeat the width matter too - the y is too wide in comparison to v, for example.

Both Z's - uppercase and lowercase - seem too rounded to be distinctive. Same goes for numbers 3 and 5, which look interesting, but do not match with the other characters.

These are just my 2¢, so better wait for more qualified opinion - after all I'm still an amateur in this field.

>>Any particular characters that don’t work in the circle/line structure?
N and V. Maybe they should be an exception.
>>Are there basic type things I’m overlooking/destroying?
"Offshoot" is when rounded letters (like O and C) exceed the cap line and the baseline as opposed to straight letters (like E and T and L) that sit on the baseline not below it and they do not exceed the cap line. I's not as noticeable in a lightweight like the one you have here but when you start building the bold version (which you should) you'll start noticing the C and O to be shorter than the rest if you don't pay attention to offshoot.

There's a million things to fix here but start with ditching the grid and fixing the widths as mentioned above.
The fact that you have a basic character set is a great start and I really like the "a" and "q".

Keep us updated.