Fun game. A couple of my criticisms of the game have been brought up already (not enough contrast between helpful items and harmful projectiles; no "magnet" power-up to make getting coins/hearts easier).
Some additional things:
1. The regular fire and fire-nomicon were automatic but the other -nomicons required me to constantly click; I didn't really notice a large difference in effectiveness and I wound up wanting to be able to turn off the other weapon-nomicons (other than fire). In my opinion, they all should be automatic unless there is a balance issue (and if you have some automatic and some not, allow them to be able to be turned off).
2. The boss battle QTEs - okay, not my favorite, but I think it was poor design that kept sending minion enemies (and allowed you to keep getting hit by them) during those QTEs. Even if you were going for a more hectic challenge in allowing that, pressing space during those QTEs should have at least induced 'bullet-time'. Dying during those sequences by minions (which I couldn't focus on because of the time limit for the QTE which, if not executed, would fully heal the boss) was frustrating more than challenging.
3. Very minor - losing the weapon-nomicon effects during boss battles even though I got it less than a second before initiating the battle.
4. Spells - interesting spells with some diversity but I'm not sure if I'd ever choose any of them other than the health spell (other than for a lark). To me they were just 'there'. Maybe add the ability to upgrade spells to make the offensive ones give a little more umph.
5. Somewhat minor - but why did the stats for how long you ran stop at 10,000 meter in the endless run? Part of re-playability is being able to beat your previous scores (one of which is distance).
6. Minor or major - I don't know if it's just me but with all of the things I was paying attention to, I couldn't see my health unless I purposely glanced at it (in contrast to the fuel which was much easier to see from the corner of my eyes). My suggestion would have been to make a health bar next to the fuel bar (the red would be contrast enough to distinguish them) - but it could just be my old eyes. Additionally, I would have liked a running tally of number for critters killed during a run to be right under the miles traveled in the upper right.
7. Achievements - not a big complaint (several games that use this type of achievement system have really ruffled my feathers) but, especially in the early game, some of the easier achievements came after some of the more difficult achievements - having the easier ones come first (and thus help you make money sooner) help with the flow of the game (player doesn't feel stuck). I don't think it was bad at all - just could have been better.
8. Endless mode was "super easy" compared to story mode - partly because I had the life spell - but also because there were no bosses. Maybe add a boss rush mode if you don't want to add them every X number of meters?