The 50% hitpoint rule.
I've seen it many times on the boards, and each time I provide the same caution. The math doesn't tell you everything.
Or more precisely...your not looking at all the math
1) Mathematically when multiplying two numbers together, the closer they are to the "middle" the larger the result.
For example: 10 x 10 = 100. If I reduce the first 10 by 50% (5). I have to raise the second 10 by 100% (20) in order to get the same result.
That is the same with hitpoints x damage = monster threat. Reduce hitpoints by 50%, you will need a lot more damage to compensate (100%...which I believe later posts are starting to note).
2) 25 damage done over 2 rounds is NOT the same as no damage the first round and 50 done the second round. 4e healing allows a party to take continuous damage pretty well...but "shock" damage is different. Take your cleric for example. He takes 25 damage the first round and is now bloodied. Uses a healing word and he's back to full...no problem. Or....cleric takes no damage in the first round...is sitting pretty. Second round takes 50 damage and is knocked out....he can't heal himself now. When you bump up your damage...your increasing the "shock" factor.
3) 4e gives a strong allowance for players to take focused fire damage. And in a game where the default assumption is you are going to fight 5 monsters at once....its a good mechanic to have. So while any single hit doesn't do that much...5 hits on the same guy can suddenly leave him barely alive....but alive nonetheless. Bump up that damage though...and suddenly you might watch your player go from full hitpoints to dead in the blink of an eye. That might be what you are going for in your game...but if its not watch out.
4) Decreasing the number of combat rounds actually strengths PCs. PCs are like fireworks...they have big attacks but run out quickly, then relying on smaller ones. Monsters just keep going and going. The faster combat ends, the less at-wills a party had to rely on....which means they are stronger then they were.
So with all of those factors, here's my take home message: START SMALL!!
A 50% change in hitpoints is a huge alteration. Try 15%, 20%....and see how that goes. Run that for a while and see if its to your liking. The greater the change you make, the more likely other things will crop up you just didn't account for when running the numbers (like the points I mentioned...and others even I didn't think of).