Welcome back to the Orkz uniqueness. While this list does have the potential for killing some "Umie"s (I can only assume by this you mean Imperial Guard), there are some tweaks that can be made to make it a more effective force.
One such thing would be the addition of weapons to the Battlewagons. Yes I know that they are just transports in your list, and that with the Deffrollas it can be quite fun to run over the enemy, but I feel that they just lack some good old Dakka. For the same amount as a Deffrolla you could have a Kannon and 2 Big Shootaz.
You can have Eavy Armor on the Nobz in the Boyz mobs if you so choose. Most players don't as usually if the Nob is taking a save the mob is pretty much decimated. If you were to than you would have enough spare points (including the 5 that you didn't spend already) to add a Waaagh Banner to your Biker Nobz giving them WS5. Another possiblity would be the addition of 2 more grot orderlies to the Bikers, as nothing agrivates an opponent more than a reroll on Feel No Pain.
Another concern with your list is that for an Ork army there is the surprisingly low model count of 72. While I can see that your original concept for your army is a very mechanized list, without the Battlewagons you could easily add in a 3rd unit of Boyz, and if you so chose hand out a couple of Big Shootas and Rokkit Launchas to them.
One reason as to why I mention Rokkits is because your list has very little potential against enemy tanks. The only hope to taking out a Basilisk or Leman Russ (as I am under the impression you believe you'll be facing up against Imp Guard) is the Biker Nobz or Ramming them with the Wagons. While these are good anti tank ideas, the Nob Bikers (and Boss) would be very good at quickly engaging static enemy shooting units like Heavy Weapon teams.
Note that while this is a critique on your list, it is still possible to win with it and you do not have to change it. These are just my opinions on how to make it better.
Peace
~OD
