Alright. Here are the traits advantages/drawbacks, taken from Games Workshops 'Codex: Space Marines' trait system. These are just my recommendations, as you may choose whatever ones you would like.
Honour Your Wargear
"May take Devastator squads as Elites and/or as Heavy Support. If taken as Elites, these squads must take the Tank-Hunter or Infiltrate skill at +3 pts/model."
This trait can be used, firstly, to include more devastator/heavy support in a non-Apocalypse tuned army. It can also be used to help show the Iron Warriors tendency towards heavy weaponry. This does not mean that they used less of the other Elites choices, like veterans or terminators. This just is another option, and depending on your preference can be used how you like.
Scions of Mars
"May upgrade a Techmarine to have an extra wound at +15 points. Any Techmarine upgraded in this way counts as an HQ selection and you may not use Commanders at all. Amend the points cost for Terminato armour to 25/15, unless the Chapter has also taken Aspire to Glory. This does mean that single wound models can be given Terminator armour."
This advantage can be used to represent Warsmiths in an Iron Warriors army. The Warsmiths were known for being able to equip themselves with servo arms, and this advantage can make that option available as an HQ. It also helps show the Iron Warriors close relationship with the Mechanicum. The single wound models being able to equip Terminator armor is less important; it less represents the Iron Warriors, but comes with the first half, and can still be used effectively.
These two trait advantages both come from the same Trait, Wise. Even by choosing both of these advantages, you are still within the "Notable Divergence" section, only requiring you to take a Major Drawback. The next advantage is from a different trait, and if you take the next one listed along with one of the ones above, then you will be required to take one Major Drawback and one Minor Drawback.
Suffer Not the Works of Heretics
"Any Tactical Squad or Devastator Squad may take the Tank Hunters skill at a cost of +3 points per model."
This can be used to represent the siege abilities of the Iron Warriors. Though "Honour Your Wargear" also gives the Tank Hunters ability option to Devastator squads, you may find this to be more useful, as it is not required and it also includes Tactical Squads. Again, whichever one you take, or even if you decide to take both, depends on your personal preference.
Major Drawback: Eye to Eye
"Although the Codex Astartes includes extensive guidance on raiding, hit and run and guerrilla warfare these techniques are not universally employed. The chapter may not be able to train in these tactics or be temperamentally opposed to them. This drawback may not be selected along with Be Swift as the Wind. May only take 0-1 selection in total from the following list: Land Speeder Squadron, Attack Bike Squadron, Bike Squadron."
This is, in my opinion, the only Major Drawback that makes sense with the Iron Warriors. It represents their tendency to avoid fast moving operations.
Minor Drawback: Have Faith in Suspicion
"....May not use Librarians. May not use any allies with Psychic powers."
Depending on what advantages you picked, you may or may not have to pick a Minor Drawback. I recommend this one because Ive never heard mention of Iron Warriors Librarians, and it is one of the least detrimental and most fitting minor drawback. Another entirely viable Minor Drawback is "We Stand Alone," which doesn't allow you to take any allies. This could represent part of the Iron Warriors paranoia in later Great Crusade times.
Remember, these are just my recommendations, and the trait system always goes with preference.
Iron Within! Iron Without!