Is there any good reason why the Disarm action takes a -5 penalty to the attack roll?
I think it must be that difficult, otherwise it would become a too common action. A disarmed PC loses much of her power (if a fighter even more), so the rules must discourage such an action, but still make it possible. In 4e there is neither the possibility to disarm, except for a fighter encounter power.
I'll assume you've played 3.5 and 4e before. Since, I'm going with that assumption, I'll also assume you know how badly disarm sucks in 3.5. I don't know what the rules in 4e are like for disarm, but if size has anything to do with it (like 3.5), then I'm sure it sucks. Does size matter for your disarm action (do larger creatures get size bonuses to resist)?
You do realize that disarm is already going to be a very uncommon action anyway. First you have to ask how often the players are actually going to face enemies with manufactured weapons. On top of that, the chance to be disarmed in return is already doing a decent job of discouraging its use, in my opinion.
For instance, there are NO attacks of opportunity, which I think make the combat very tactical.
Ah, see I hadn't noticed this. Interesting.
Second, feat and talents that slide, bull rush and move opponents are much less common than in 4e, so the battlefield (see, miniatures and grid) is less fundamental.
Which, personally, I do like. Being able to move opponents is nice (since 3.5 Bull Rush is really the only common ability, but it sucks), but 4e went NUTS with it.
Still, if you have some ideas that will differentiate my system from 4e, you are really welcome to explain them. H20 does not want to be a clone (it would be illegal too), but a stand-alone system. The draft is similar to 3.5 and 4e to create a common ground where to experiment.
Well, 4e and 3.5 combat statistics aren't drastically different, so going for a middle ground is naturally going to resemble them. There are differences to be sure. Probably enough to cover your backside.