I've run the demo on roll20: Some thoughts/Questions

Good observations. Explosives do require an additional step to resolve, but Arcanet is right in that they are designed to be powerful - and rare. I included a few in the demo because they are fun, and I admit the rules are a little loose in explaining how they work. I'll make sure this is cleared up in the manual, but for now let me clarify:

You can attack a square, which doesn't try to avoid your attack, and thus it is an "unopposed" attack.

Anyone in the blast radius typically doesn't get to flip their defense to avoid the blast. However, each affected character who was wearing armor should flip a card (the GM flipping for each NPC) to see if their armor negated any damage. Also, some explosives have various effects which affected characters have to flip for, such as flipping build to avoid being stunned. In such cases, just flip one card for your ability and use the pips on the same card to see if your armor saved you.

There is of course the potential for skills that will affect your character's chances of surviving an explosion, but for the most part explosions are very dangerous for anyone who happens to be standing too close and the rules are intended to reflect that. I hope that clears things up.

Also, as far as the decks/player hands obscuring the table, you can use the zoom function of your browser to adjust the size of the on-screen elements of Roll20. The official module has some custom UI elements that help cut down on this issue. Be sure to use the zoom tool (in the toolbar on the left of Roll20's UI) in order to size the map the way you want it too, and be sure to teach your players to do the same - it can make a big difference. Also, in the settings I turn the player avatar size to "Names Only", and again I recommend your players do the same to get the most screen real-estate possible. It's a balancing act, but thus is the current state of Virtual Table Tops.

Wow, thanks for the wiki link! I'd never seen that before (somehow).

And no, I don't add add checks to grenade damage, unless you throw the grenade directly at an opponent instead of the square they are standing in. In that case they get to flip their defense (which may save them the addition damage from the checks, but they could still possibly be caught in the blast depending where it goes off, even if they avoid the grenade. Everyone else in the blast radius is affected as normal.)

Thanks for the feedback, I'm super impressed with what they have done as well, it makes a huge difference. That being said, I believe there is still some interesting things in store for the future decks on Roll20. The Roll20 guys are great, I GM'd an adventure for them to show off Simple System during the Kickstarter Campaign and we had a great time. I think liking the system had something to do with their enthusiasm for adding deck features (at least I hope so!). In the end the deck improvements will benefit a lot of games, and they were already interested in adding features, I think Simple System was just a good opportunity for them to do so.

You can adjust the displayed card size in the deck settings. By adjusting the dealt card size to 300x425px I found they were quite easy to read when placed on the table.

EDIT: If I read you correctly, you would like to enlarge the cards in the player's hand as well.  Agreed!

I do use the hearts and skull cards, so I like to see them, but I agree it can clutter the screen a bit. It also helps because I have a mechanic where I give one player a "badge" card to signify that he/she is the squad leader.

The "Z" button thing annoys me because it masks out the screen when it happens. I like having the card big and readable the second it hits the table.