First of all I found Serious Sam Double D a very fun game. There are plenty of secrets to discover, the gun stacking ability is pretty fun, the enemies bizarre and bosses are huge. I think it does relatively well to capture the crazy essence that is a Serious Sam game.
From a design perspective, there are some minor issues that could use a bit of tweaking. A lot of the issues centre around the game physics and certain design decisions. All of these are minor and don't really get in the way of a cheap, fun 2D shooter.
Controls and Jumping - Controlling and jumping isn't bad in the game, but it lacks a weight to it compared to other platformers like say Megaman or Contra. Some reviewers complained about this problem saying it completely ruined the game. I think there is some issue to it but not to the extent of spoiling the game for me. The jumping pad does ad an extra dimension to dodging but I wondering if putting in a double jump or super jump might have been a slightly easier option?
Gunstacking - Gunstacking and swapping gunstacks is fairly intuitive however I found the gunstacking a bit underutilised. Yes you can stack lots of guns with each other but all too often I found myself pretty much gunstacking all the shotguns or all the tommy guns or all the rocket launchers. Only very occasionally would I use the chainsaw or greande launchers or flamethrowers.
Enemies tend to be quite powerful so you pretty much need all the firepower you can stack. I guess what I'm trying to say is that although gunstacking is fun, the significance of gunstacking choices diminishes at times so I'm left having to use the obvious choices like 4 shotguns rather than mixing and matching.
Also, when you gun stack you can only gun stack weapons that you've found in one gun stack. For example, imagine if you've found 2 shotguns and 2 tommy guns each gun can only be in gunstack 1 or 2 or 3. You can't have your shotgun in both gunstack 1 or 2 at the same time so when you swap, the same shotgun appears in stack 2. The system just doesn't allow that. In gameplay terms, I found this broke the flow of the game. In the early parts of the game when I had fewer weapons, rather than swapping between stack 1 and stack 2 and stack 3 with the keyboard, I ended up continually entering to the gunstack screen and reconfiguring my existing stack to manage the small number of weapons I had.
From a game design perspective I wonder if reducing the number of guns to a more manageable 4 but increasing the damage might be a better idea. Also having some sort of combos system or bonus system where using certain configurations might trigger bonuses (i.e. 2 lasers = more damage) might be helpful. Some might argue that having 2 shotguns and 2 laserguns firing at once is your combo system and they might be right on further reflection.
Dodging enemy bullets can be difficult - Because of the 2D side scrolling nature, it can be difficult to dodge some enemy bullets. Yes, you are relatively manoeuvrable in the air thanks to the floaty controls but the bullets are too closely spread in many cases. It's not a deal breaker but I think spacing the bullets and increasing the zoom out might have been better.
Artwork and animation- This is personal opinion but I think the artwork and animation should take a different path. I appreciate Serious Sam is not really a serious game but I find the 'paper doll' feel of the graphics and the lack of good animations an area which could be improved on.
Zooming and character size - When you look at games like Metal Slug or Contra or SHUMPs like R-type, the characters or ships tend to be super deformed or smaller than usual taking up relatively small amount of screen space. The camera in this game will often zoom in and out to show how large the enemies.
From a game design perspective I wonder if it might be better to stick to just one perspective and design the game around that single perspective. This might sound restrictive and a minor detail but I think it might have forced the designer to carefully consider certain aspects of the gameplay such as the size of the bullets, the space required to succesfully dodge incoming enemy fire and the floaty controls as well as creating more integrated experience. On several occasions they zoom just wasn't quite right.
No cannon - One of the signature weapons of Serious Sam, the cannon is missing from the game! A serious Sam game without the cannon? It's like accidentally leaving out the shotgun! I'm sure it wouldn't be too overpowered...
Overall a game that does convey the Serious Sam mayhem most successfully. It isn't quite Contra or Metal Slug level of polish however which is a real shame but the game is fairly easy to like and is very enjoyable.