I don't think people can really complain if the Daemon Prince gets nerfed.
Unless you were running a themed list or really didn't want to use them there was virtually no reason not to run a pair of them in pretty much any army list. They were that much better than the Chaos Lord and Sorcerer Lord it was really rather sad. "Hey, for 20 points more than a basic Chaos Lord, I am AP2 (as opposed to AP-), a monstrous creature, an eternal warrior, WS7, S6, T5, W4, A4, etc.
The rumours don't really say their actual statline and so on will get hit, from what I could tell it is more that they will be as expensive as they are in the Daemons codex (i.e. about 30-50 points more when you upgrade them similarly), but like the Daemons codex, will have some crazier wargear options than they do in the current CSM codex.
I have a pair of them, and I always use them. Frankly, it's getting tiring. I have an awesome Chaos Lord/Sorcerer Lord that looks like Ahriman and a Sorcerer Lord in Terminator armour, neither of which I ever use because running them makes no sense in a codex that already struggles against most others. Not that I'm actually really concerned about winning - the Daemon Princes draw amazing amounts of firepower and it is pretty funny to watch them laugh it off. Hell, the only non-Daemon Prince HQ that was worth his points comparatively - Kharn - has been hit pretty badly by the recent FAQs. No murdering entire squads before they get to attack sucks ***, especially when you strike at the same time as a power fist that will still kill you outright.
This is what I love about the new codex rumours. The special characters all sound delicious - especially with points reductions - and as a Tzeentch player I am very excited about Ahriman now. If he is indeed Mastery Level 4, and can swing a better version of Smite in addition to his 4 psychic powers a turn, all at a reduced cost, I think I will be running him all the time. It's at that point I can say to Spess Mehrines - 'Lol, my psyker's better than yours'. The best part though is the difference between the Chaos Lords/Sorcerer Lords and Daemon Princes - the DPs are obviously the most powerful combat wise, but the other two buff/change your army by affecting your force organisation, as well as providing other buffs (i.e. an undivided lord provides leadership bonuses and reserves manipulation).
Still.....the Thousand Sons sounds insane now. If the rumours from that Mysterious Guest are true, Thousand Sons won't be ridiculously expensive anymore. 230 points to get a 9 man squad and not have to pay for the Sorcerer's psychic power will be awesome, as oppossed to the 254 points minimum from the old codex including Doombolt. That and now Thousand Sons get Soul Blaze and are only Slow and Purposeful if the Sorcerer dies. With the new potency of psychic powers and, being Chaos Space Marines, the probable addition of some very nasty new powers, no longer will we be the black sheep of the cult units (points-effectiveness wise).
But man.....being able to mark vehicles....hell yes! And dedicated transports get them for free matching the squad they carry? Daemonic possession turning your vehicle into an actual daemon (that's the impression I'm getting) and thus receiving a 5+ invulnerable save? Consuming your own models to repair/regrow weapons and so on said vehicles? A flying dragon that mauls other flyers? Close combat Obliterators? Daemonic raptors? Cheaper units? Another Land Raider variant? Better Defilers? Possessed actually getting to buy daemonic gifts? Eye of the Gods table for ALL characters? Abaddon back to being AP2? Kharn back to murdering squads before they attack? Cultists? Plague Zombies? A new Daemon Engine? Being able to MARK Dreadnoughts!?
It just sounds too good to be true. But by the looks of it IS true and it's coming our way in September. I can't wait. This codex sounds amazing - being a Phil Kelly codex, is that really any surprise? That last one makes me smile - I have a Thousand Sons Sorcerer Dreadnought converted from two Furioso Librarian kits (I have both force weapons, one on each arm - it is an awesome, awesome model). Whilst it's not confirmed, the guy did say that Dreadnoughts can have marks, and given what he said about the Khorne one (i.e. it gets blood talon-equivalents and becomes a meat-grinder), it isn't too much of a stretch to think we might get psychic Dreadnoughts too
Excited rant, over and out. Oh, did I forget to mention Dark Apostles and Warsmiths?