Improved Soul Fire is the most hateful, horrendously god awful mechanic in the world, and makes me never want to play my warlock again until they fix it.
The 15 second cooldown on the effect is complete lunacy, going for beyond the "challenge = fun" aspect and simply becoming hideously frustrating, and making the entire thing hinge on having perfect latency and being 100% perfect with your timing. When I say that, I mean "you need to time your cast based on both duration left on the buff, factoring in your latency, as well as the travel time of the spell because if you were standing abit closer than last time then it will not re apply the buff and you are left ****ing in the wind hard casting yet another Soul Fire."
It is relatively annoying for Destro, for Demo however it is absurd, single handedly making the spec more difficult than anything else in the game for now, not only are you watching all of your timers, including Hand of Guldan Cooldown, Molten Core procs, Meta cooldown timers, planning on how best to get into melee range because Immolation Aura and Shadowflame need to be used on CD to get the best dps and managing your pet, but you are also watching this ridiculous haste buff which is totally essential to your dps, but it also the most fiddly buff to reapply in the whole game.
Removing the internal cooldown on the effect would make it better, but I am not finished in this rant about how utterly horrendous this is as a game design mechanic.
It is ESSENTIAL, for ALL specs. Yes, even Affliction. You read that right, Affliction, a Shadow damage DoT based spec now has to hard cast a soulfire every 15 seconds. In the execute phase, where watching Shadow Embrace stacks, DoT timers and your cast bar to work out when best to clip drain soul to re apply dots, this becomes utterly nightmarish, as you also have to plan for 2.5 seconds of hardcasting soulfire time. If you could refresh the buff whenever you want, that's fine, as you can do it at an appropriate time. But you can't, you have to refresh it at precisely the right time (with about 1-1.5 seconds' margin for error), and if that happens to line up with all your dots needing refreshing and Shadow's Embrace being about to fall off - LOL TOUGH LUCK AFFLICTION LOCKS!
The problem is not only the clunkiness of the mechanic, but also that it forces warlocks to subspec destro. There is no choice in the matter, no matter what spec you are - you WILL be putting enough points into destro to max ISF, because 15% haste is ludicrously good, and if you don't spec into it you are being hideously sub-optimal.
So now all Warlock damage is balanced around keeping this 15% haste buff up, despite it being the most difficult buff to reapply in the game.
Words simply cannot fathom how awful this mechanic is - which probably explains the 23 page thread on the official forums about it, which has some extremely good players from extremely good guilds all saying how moronic it is. ISF needs to have its cooldown removed, and it needs to be brought out of reach of both Demo and Affliction by moving it further down the Destro tree. Another reason why it is such a horrible mechanic is that Soulfire for Demo is their Execute, it makes zero sense to be having to hardcast Soulfire every 15 seconds, despite it losing alot of it'spotency because you won't be casting it under the effects of Decimation - it's just a massively counter intuitive mechanic.
The best thing about Bloodlust now for locks is that it is 45 seconds where you can forget about that ****ing buff because the two effects don't stack and Lust removes it.
Also, Demo won't overtake destro or affliction, Simcraft has shown us this, see here:
Sims done with both pre raid and raid gear. Those sims are done on standup fights with ideal conditions - so in practice demo will be even worse than Affliction or Destro due to the sheer difficulty of the spec.