Woo, attention!
I think that's everything. Hope I didn't bore you with my blathering or anything. 
That sort of blathering is pretty much what I've been waiting for for 2 days.
1. As gorgeous as the explosion and smoke effects were, I often found them obscuring my vision. Perhaps you should make them partially transparent, say with image alpha? Another solution (that is, if this constitutes a problem) might be fading the effects (particularly the explosions) a good deal quicker after they've made their initial appearance.
I'm gonna get rid of all the fancy halftransparent stuff in it anyway since this is an old project I deciced to pick up again and pretty much change the look of it so I won't introduce more fancy halftransparent stuff.
If I made the explosion animations play any quicker (that sideways one is already playing at full 60 fps!) they'd be pretty much nothing but quick flashes.
Even quicker than they are right now!
I don't know if you've noticed but this game is pretty much all about explosions and madness but yeah, I'll make a more appropriate hitspark animation since this is the biggest view obscurer right now.
There are smoke effects in there? I haven't even noticed.
2. A type of signal to indicate what area of the screen enemy ships will start swarming from next would be a peachy addition. A typical flashing '!' of some kind would do. I feel like this would keep the player from becoming overwhelmed and give them a few adrenaline-pumping seconds prior to a massive onslaught to adopt a new battle strategy. Mind you, this is really only a necessity in my eyes when dealing with the mass deployment of smaller, speedier foes. The bigger ships really aren't much of a threat in this manner since they crawl along at a modest pace and give the player adequate time to make some evasive maneuvers.
If there were enemies that come from behind or above or something then yes.
Right now the only strategy you can resort to is shooting everywhere without losing your breath because if you stop shooting, you're pretty much dead and there's pretty much a consistent stream of enemies so where should I place those warnings anyway?
It'd be like telling a person standing under a waterfall 'hey, watch out, there comes water from above!'.
But yeah, as the enemy variety increases, there will be some enemies that'll require a heads up so that'll be considered.
3. As much as I enjoy thrashing wave after wave of mindless, generic automatons, I'd encourage you to go all out there as far as the future environments and enemies go. Why not have your pink dragon protagonist romp through a futuristic forest inhabited by airborne mushroom robots, mechanical wizards, and malevolent fauna? The sky and your imagination are the limit I suppose, but I believe that this game could potentially combine the pulse-pounding action of a side-scroller with a slew of creative elements to further its replay value.
You haven't seen anything yet *sinister grin*