- usually very slow scrolling which makes the added environmental factors feel more static than something to be very careful of
- health bars in conjunction with unavoidable damage. Basically it's not the fact that the game has health bars in themselves that make the euroshmup what it is, it's the fact that they add the mechanic because the devs are unable to come up with thoughtful level design and patterns and inevitably introduce damage that is just not avoidable.
- more time spent on adding crappy gimmicks like adding mechanics to purchase weapons in lieu of polishing the core gameplay
- thoughtless placement of enemies, absence of distinctive flow/interesting routing to think of across levels, which ties to point 2 with healthbars. Barely any attempt at making level designs beyond of having to kill a specific turret or large enemy occasionally while the screen scrolls slowly. In japanese shmups, learning routes to preemptively kill enemies is vital. There are full sequences where things get very hairy if you're not already pre-positioning to hit before spawns.
- no concept of pacing, large quantities of enemies with inflated hp coupled with little player power progression (oh you can customize your weapons and buy new ones! but things will still die slowly no matter what).
- controls that feature gravity/inertia
- smudgy pseudo-realistic busy artstyles that detract from the focus on the core of noticing patterns and dodging them.
euroshmup wankery at its finest :
brown shitvomit covered in fog
No game exhibit every single symptom listed, but if you hit more than 3 chances are it's just euroshmup trash.
The TL;DR is that euroshmups are made by developers who try hard and stray as far as they can from the very essence of shmups : shooting and dodging. The design philosophy is to keep throwing stuff at the wall and hoping it sticks.