The main problem with deff dreads is that they exist in 6th edition for around 100 points.
Dreadnaughts have changed roles in recent years, and deff dreads haven't caught up with the times. If you look at the popular configurations of the good dreads out there then you will soon realise that they are mainly shooting roles. 2x Autocannons, psyflemen, Las + missile. All designed to screw over light to medium tanks or to destroy medium armour save troops. Ork dreads just can't do that. Sure, we can kick out alot of firepower for one model, nobody is going to sniff at 6 strength 5 shots a turn, but it is our ballistic skill that really makes the model suffer. Throw in that grenades are far more deadly for dreadnaughts and overwatch is a bitch and you may never actually see close combat with the expensive tin can.
And lets not forget one of the major points here: You're walking. There is no sneaky out flanking, no drop pods, no teleportation. If a deff dread wants to do what it does best it's going to have to trundle up the table at a measly 6 inches a turn. That's rather pathetic. Sure, it can run D6 every turn if it wishes, but then you're forgoing a shooting phase or two. Or you could upgrade to two extra close combat weapons in place of the shooty stuff, but then you're making an already costly model even more expensive, and that's just not a good idea. The only combat dreadnaughts out there that actually make it into combat have some way to cause pressure on the enemy units.
And last but not least it's time to look at just what they take up in the army. We've already covered that they cost in the region of 100 points for a model that is only really any good in one phase (and usually one turn) so they naturally cost an arm and a leg for what amounts to little more than a distraction, but they also take up a heavy support choice. That slot can be better spent on Battlewagons or the nicely buffed big gun units. bot do the role of the Ork Dreadnaught better. A battlewagon with deff rolla filled to bursting point with boyz will make mincemeat out of most things, whether they are equipped with shootas or choppas. The big guns do what most modern dreads do and provide a good amount of tough, somewhat reliable, hitting power at long ranges. As far as I am concerned, those are the only two options for heavy support in our codex, and the Dreadnaught doesn't come close to contesting either. So if you take a dread then you're detracting from the good stuff that is still available in the aging book.
And yes, I know that you can take up to two dreads as troops. But that sufferes from the problem of taking away slots for shoota boyz, one of the little golden wonders that we have in the book. Ork lists are usually struggling for troop slots so every dreadnaught takes away another trukk, mob or grot squad. Hell, if you go dreadnaught heavy then you're looking at ditching scoring Nob bikers. Which is silly. Whilst Nob bikers aren't to everyones tastes,and they do have flaws, they are one of the most potent units that we have in the book, keep using them whilst we can.
But this isn't a thread about a rant against Deff dreads. It's about finding a way to use one of the most Orky units ever! If you are wanting dreadnaughts theres two ways to try them: Lots and one. Now, when I say one I don't intend to go against what I said above, what I mean is if you find yourself with 100 points and a heavy support slot burning a hole in your pocket then pick up a dread. Keep him as cheap as you can, add grot riggers, and have him run at the enemy general. The thinking is thus: your opponent has a choice to make - to shoot the battlewagons and boyz that a streaming towards his lines putting out a torrent of bullets, or to shoot the little dread that is not a threat for a turn or two. The idea is to take advantage of target priority and have the dreadnaught ignored until he has a chance to insta-gib a squishy general with his strength 10 attacks.
The other idea, if you wanna go dread happy, is to build a full list of them using imperial armour 8 and just let sheer weight of deff dreads overwhelm your opponents heavy weapons.
Hope this helps.