Yoomp! is fiendishly addictive. Of all the homebrew games I've seen in recent years I think it's the one I'm most impressed by. It's a winner on so many levels - originality, technically and of course most importantly, playability. It really does play fantastically well.RGCD's Heavy Stylus interviewed the developers here. Just scroll down to the end of the review, it's right underneath.
Though I'm sure a creative coder could've made a better fist of it, I'd maintain that Green Beret is a tough ask on the Atari 8-bit because of the way colour and moving objects are handled.The problems arise from having to use Player-Missile Graphics (PMGs) for the player and enemy graphics. These are in limited supply (4 players, 4 missiles - though the 4 missiles can be combined to create a 5th player). Players are only 8 pixels thick and have only one colour, and missiles are only 2 pixels thick and also have just the one colour, so just for your player sprite alone you're using a player graphic and an expanded missile PMG to add available player PMGs already. The enemies have to be made out of the remaining 3 players and 3 missiles, so most of them are single colour, though the 2-colour ones are made the same way as the player (1 player PMG + 1 missile PMG). You can see when the character lies down how limited the 8-pixel width is - it looks horrendously blocky because in order for a single player PMG to be that length, it's having to be expanded to quadruple pixel width. On the C64, that problem would be solved by just using two sprites side by side - with 8 sprites, all of which can have up to 3 colours, you have a lot more flexibility (even without multiplexing - which doesn't actually work all that well in games that need all the sprites primarily in one horizontal band - see Target Renegade, for example).One advantage that you do get with the A8 is that PMG height is the entire screen - so what you could do is paint two enemy characters on the vertical "strip" and instruct the ANTIC chip to change the horizontal position of the strip so that the enemies that are using the same PMG can move independently. I don't know that they took advantage of that with A8 Green Beret - in any case it would cause nightmares with enemies going up and down ladders.The other option of course is to forget about using PMGs and use "software sprites", i.e. make the sprites out of character blocks. Problem with that is that even in character mode you're limited to 5 colours. By default that's a screen-wide limitation, but thankfully the Atari's ANTIC is set up to very easily allow the screen to be split horizontally so that you can have bands with a different set of 5 colours. So you can get tons of colours on screen for games like Frogger where each row can potentially have a different set of colours. But in a game like GB where you need loads of objects - both background and characters - all on the same row, if you try to create the players out of character graphics you're going to be limited to whatever is behind them horizontally - in this case, black, red, white, blue and purple. So it's a difficult limitation to work with in this case.All in all, it's the lack of PMGs to work with that accounts for the rather barren nature of the A8 conversion. I have always thought that with games designed specifically for the Atari in mind (Yoomp!, Rainbow Walker, Crownland and Dropzone, for example) the machine comes up trumps. Try to crowbar on to it something that's not designed for it and you've got problems.
Here's a slightly off-the-beaten track one for y'all: Kult by ASF. It's in the Atari's hires mode, hence the lack of colour. However, it is very responsive and therefore quite playable. There are a lot of A8 games from the late 80s produced by Polish studios - I expect Poland became a bit of a dumping ground for A8s after it was decided that it wasn't commercially viable to support them in Western Europe. Likewise, the Commodore Plus 4 appeared to enjoy a late surge of life in Hungary in the late 80s.