If that is the goal, Statesman, you need to go back to the drawing board on this change. For most people, it is not that big of a difference. Tanks and Scrappers can still take on bosses. Damage dealing controllers can do it too. Blasters and Defenders it varies alot.
The problem is, that Bosses will just insta-kill you if you screw up at all. This isn't fun or challenging. If one of my holds miss, I die. Generally don't even get to see the animation of the attack that killed me, just dead. If I do get the boss held, I sit there and chain hold it while my pets kill it.
Scrappers and Tankers it just takes alot longer, and if a Def based character gets a little unlucky, insta-death.
And, finally, the big problem is that bosses are not worth the experience you get for killing them. Right now 7 minions or so is the same xp from 1 boss. I can kill 7 minions without taking any damage in a matter of seconds. If you pile them all up, it doesn't take significantly longer then killing 1 minion.
A boss takes about a minute, at least, to kill. The math just becomes not worth it.
If you want tougher bosses, make them smarter. Make them summon allies, give them pets, give them neat abilities. Make their attacks penetrate resistances, or give them cool debuffs. Lots of things you can do.
Making all the crunchy classes have to pray they don't get one-shotted is not fun. It is not more challenging. I agree with an increased difficulty post 25, but this is not how to do it.
