The Play Phase

When a spell is on the stack in the Play Phase, you should not be able to play effects or monsters.

Right now you can abuse it with, say, 16, FoH, and a 3-cost creature (mana as appropriate). It gets countered? Play the creature. Wait a turn and FoH again. Repeat until oponnent runs out of counters. Right now, this is like a "hidden strategy" that only experienced players know about.

I don't know if this is possible to code, but this seems like something noone would really disagree on, and it would also make counterspells have their intended effect more often.

EDIT: Jeez, a spelling error....

Another possible solution would be to deduct mana from your totals when you add the spell to the stack. That would prevent accidents where you stack more spells than you can physically cast. Additionally you could not do something like:
Play Alien Lawyers
Opponent Players Crack Under Pressure
You play Thundermancy
Which (I believe) would have the same effect as playing a monster to drop your mana totals.

I dunno, it's sort of annoying, but not too big a deal in my opinion.
Was it Whomper's deck which prompted this post? :)

Clickwars bother me. Non-clickwar use of S-speed abilities does not.

As for your specific example, if my FoH-attempting opponent is forced to play out a creature and not have cast FoH I don't think I consider that a straight-out win for them. If my opponent thinks wiping the board is a good idea, I suspect I will enjoy that extra turn. And that's one less creature they can play post-FoH. (Plus now I know there's a FoH available and can play/sac appropriately.)

I agree, we should nerf the play phase! I can't tell you how many times my opponent has used it to win!

People argue that counter spells as they are need to be nerfed. The whole play a card to avoid getting you spell countered is one of the defences against that arguement.

Though I wouldn't mind the mana amount to still be deducted for failing to cast a spell (with the mentioned trick). I.e. FoH would deduct 15 for failing to be cast in the prime example mentioned but the person still gets to keep the spell.

So you'd give him negitive mana?

Negative mana==bad.
How about the spell fails, the same as countered?
Alternatively, the casting of a spell during the play phase could be a tactical maneuver.

Honestly, while I like the idea of tactical maneuvers, I think it makes sense that the spell would fail to resolve.