I started with the basic rules when playing with my son too, Little_Odo. I hadn't really examined the "Master Rules", but it was he who kept wanting them (so that his dragons could fly and use their special powers). After reading through them, I found that they really weren't very much more complicated than the basic rules at all. You may have read them already, so know what I mean. Basically, it adds the use of the special powers, most of which are very simple to use, and the concept of wounds, which simply involves placing wound tokens on the cards until they equal the characters life points, at which time they are destroyed and removed from play.
My son loved the basic play, but it really came alive with the "master rules". The fact that his characters can be attacked, fail the defense roll, and still live to exact some revenge appeals to him far more than the instant dismissal in the basic game.
I know what you mean about it taking over the dining room table. As I type this, I am sitting at ours, with a HeroScape war raging across the rest of it. I have eked out a little space for my laptop amid the carnage.
One of things I appreciate with this game is that expansions are by no means required to fully enjoy the experience. I get more because it's just so much fun! The trees, road tiles and bridge (from the Forgotten Forest expansion set) are a great terrain addition. Although they don't add much (only 5 trees, 1 bridge), I like the look of trees on the field.
Of course, I feel we simply must have the castle set, and one or two copies of the ice (tundra) expansion. And then there's the Roman soldiers, and the Zombies, and.... well, there's a lot.
They are releasing an "expansion" in the next month or so called "Marvel HeroScape". There is no news of it on the official site, but
www.boardgamegeek.com has a fair bit on it, including photographs from a gaming convention showing off the bits and some of the figures. It will features 10 Marvel superheroes/villians and a few scenery bits more suited to creating an urban evironment. They are saying one should not mix these figures with the regular HeroScape lot, keeping it an all superhero affair, but the terrain tiles are still useful.
I still want to play heavier games with my brothers, friends and wife, but I will seldom turn down a game of HeroScape. It's the best excuse to play with toy dinosaurs, dragons and soldiers I can think of. Note, I do not consider the GW models toys at all, they are miniature models. These plastic goodies in HeroScape are toys, but toys with rules!