Ok so I just played through the entire game (not full out exploring but not rushing either) on original mode, and I tried out arcade mode to see what it's like.
Arcade mode looks pretty good. Basically you get a timer and you are looking to increase the time to keep yourself alive which is a lot of fun and a much more hard core way to play the game. Also playing for score I'm sure would be amazing. There are literally TONS of secrets in the game and many of them are really well hidden. Really excellent level design all around. When you continue on arcade mode you start over at the beginning of the level, while in original mode you start at a stable platform nearest to where you died.
I was keeping an eye out for the signs to see what exactly is going on. It looks to me like the large GO! signs are fine and the large EXIT signs (rare) are also fine. The small red exit signs that point left and the ones that point up have inverted text, and it seems to only effect the first 3 levels. By the beginning of level 4 (or maybe even as early as the end of level 3) this problem corrects itself and all the exit signs are fine.
I noticed some interesting things I hadn't noticed before. You can actually kill smaller enemies by jumping on them (a la Mario). I tried jumping on all kinds of things to see what would happen, and I found that if you jump on top of those small swarming helicopter enemies they actually kick you up high into the air which I thought was really cool (you can use this technique to reach some secret areas). There is also an enemy later on in the game that is safe to stand on (it doesn't move around) though safe is a relative term since it is trying to kill you all the same. Starting on level 4 the levels become MUCH more difficult and tons of fun. I can't stress enough how pleased I am with the overall level design throughout the game. There is a really good variety of terrain and level mechanics between the levels which keeps it very fresh, and the level design really is amazing. Along with the music and the great enemy variety the stages themselves are tremendously enjoyable with great atmosphere.
I found myself using the C beam more and more in the later levels, almost to the exclusion of the regular weapon, since you get swarmed a lot. I'm tending to agree now with other opinions that it's a little too powerful (with a caveat). The length needs to be cut back slightly (maybe 2-3 notches). How to adjust the power though is a tough question, since it needs to be done the right way. The fact that the beam can instantly kill the smaller stuff is working totally fine as is. The problem is that it does too much damage too quickly to bosses, which eliminates having to use a more strategic approach to defeat them. For example if you find a relatively safe place to stand during a boss fight (not as hard as you might think considering how long the beam is) you can just set the beam at the appropriate angle and finish it off very quickly, maybe only having to move once or twice during the fight. Perhaps the best and simplest solution is to leave the power as is but make bosses immune to this weapon. Doing this would fix the issues people have with the beam being overpowered (since really it's only overpowered versus bosses) and also significantly improve the boss difficulty issue (though the fights themselves still need to be made HARDER).
The boss/sub boss fights are all too easy throughout the entire game. The difficulty of the boss fights do go up slightly starting on level 6, but they are all still way too easy. The only boss fight I thought was okay was the final boss, but to be clear, the difficulty of that fight was about right for something like a level 3 boss, it was by no means anywhere near hard enough for a final boss fight. Having said that I actually enjoyed the final boss fight tremendously (I won't spoil it), but want to see more like that. All the boss fights should be similarly dynamic. While I'm fighting the boss, throw a bunch of regular level enemies at me and also massively increase the boss' firepower. Mix it up between destructable bullets and non-desctructable ones. It's disappointing when you get through a tough level where you died a lot and then you can kill the boss on the first try without having seen the fight before. I know you guys know how to do this since the Fast Striker bosses were great.
The first 3 bosses should be relatively easy, but you shouldn't be able to kill them on a first attempt unseen. It should be sufficiently long enough to take maybe 3 or so deaths before you learn the fight, but then once you know the pattern you should be able to kill the boss every time. The second set of 3 boss fights should be harder. It should be a challenge (but doable) to get through the fight even if you know the patterns. The third set of 3 boss fights should kick your ass left and right. It should be HARD to kill them even if you know the fights forwards and backwards, and should require a little luck.
I thought the ending of the game was nicely done and was enjoyable (I won't spoil it either). All around great job and an excellent game.