Okay, I'm in World 7 now, the difficulty went up somewhat though level size is fairly uneven, some are short, linear affairs while others are wide, exploration-y. There are no themes to the worlds, they all feel the same. The boss is always the same dragon, too damn easy (then again the game is based on an SMB1 clone so having the same boss in every world with minor differences is expected, right?). He gets a bit harder every time (or at least that's what the manual promises, while he gets more patterns and HP that doesn't really make him much harder) but you can still pretty much cheese him by jumping on him again as soon as his hitstun ends (fairly easy to estimate). While I haven't done more than 2-3 of those jumps in a row it still makes him a joke. Doresn't help that there are two blocks containing punk balls in the arena so even if you do get hit you can recover fairly quickly. I think they should have dropped those to force the player to beat him without much extra health. I've already accumulated so many extra lives (because gems are dealt out in huge numbers) it doesn't matter when I die and I suppose they could have dropped the lives system completely had the gems not been an important part of the GS appearance. Guess they could have turned them into some form of currency though. The only thing you seem to lose on a game over is any mid-level checkpoints (or at least that's what the manual says, never seen it myself), considering the size of the levels they could easily have made you start the whole world over. Oh and there's a score and a highscore table, seems pointless though since you can just replay any level you want and I don't think it just saves level scores and adds them up. Really doesn't look like there's any incentive to replay the game so far. Going for 100% is so easy that it can be done on the first run. The red gems are hidden but I found them pretty easy to find anyway. You often have to go through passable walls but there tend to be hints which ones are passable and if it's just the visible cave full of gems behind them.
I really think they should have separated the fireball from the punk ball (that way you'd also have more reason to search through blocks, once you have the punk ball all you get is gems anyway) and dialled down the gem numbers. The platforming might also be easy because of the collision handling, Giana is fairly wide and you can easily land on a very thin pillar even though it looks very difficult. I've had a case where I ended up standing on the corner of spikes too, may be a collision glitch that could be abused for a speedrun.
The game may have been intended for younger audiences, a bit weird considering it probably appeals more to the nostalgic C64 gamers. Much easier than the original and not just because you can take more hits, some levels include segments from the original game and in the one depicting level3 from the original the bouncing ball that always gave me trouble as a kid got removed (and it was in World 6 or so, the THIRD LEVEL from the original, it had something like 40 levels!). Occassionally I got the feel that they backpedalled on something, e.g. one castle level had a gem between two very hard to eliminate enemies with a cola bottle dispenser and the clue that the bottle can push enemies but the gem was blue with a red gem placed in the open a bit behind that.