They scale off how much of your primary stat you have, so ATM (for TWing at least) you want to use neck, rings, trinkets, gems and enchants that don't have primary stats.
So this changes a few things and it depends on what kind of class you are. Unlike in Vanilla/Classic, things now are pretty standardized (ie boring) such that if an armor type can be used by a class, it'll have the primary stats for all specs of that class, specifically:
- Plate: Str/Int
- Mail: Agi/Int
- Leather: Agi/Int
- Cloth: Int
Now, as we know whatever your armor type is, you can use "lighter" armor if you want. So Plate Strength users have the most options because likely none of the other 3 armor types has STrength on it. That gives you a lot of options.
Also, even though stats are kinda normalized, they're sometimes not. So a level 11 TBC dungeon drop might be +4 Agi/Str/Int but sometimes it's +3 or +2. And there doesn't seem to be a reason why.
Lastly, on older gear secondary stats absolutely aren't normalized. Some items have none, some items have a ton.
Weapons are a little more interesting. There are a couple of interesting options. Shamans, for example, have some interesting options to reduce their Agi/Int, most notably
https://www.wowhead.com/item=24461/hatebringer and
https://www.wowhead.com/item=27986/crow-wing-reaper (20+, this one has Agi but not Int). There aren't good +Int weapons with 3 sockets so with wanting to reduce primary stats, these two weapons become far more tempting.
Another thought: TBC has some interesting set bonuses that shouldn't be slept on. Many legacy bonuses are inactive but not all, for example the Desolation bonus (eg
https://www.wowhead.com/item=27528/gauntlets-of-desolation) is +17 Crit from a 2 set. That's a lot.
I've tried to find +Primary trinkets with Equip procs (rather than on-use) because I'm forgetful and lazy. We probably want secondary stat trinkets instead (equip or on-use).
https://www.wowhead.com/item=28830/dragonspine-trophy has always been good stats for the level (30+). There will be others. TW trinkets tend to be fairly formulaic. But some stand out like the
https://www.wowhead.com/item=136715/spiked-counterweight . Legion TW has been a good source for trinkets in general.
Rings/necks usually come in two varities: with and without Primary stat. Primary stat jewelry has lower secondary stats. You probably want the non-Primary versions and ideally the versions you can add 2-3 sockets to (3 to DF necks from DF JC, 2 to TWW seasonal rings/necks but you can only get the sockets from the AH below level 70).
We probably need to be more explicit in the (excellent) F2P Baseline Guide because there are a ton of single and double secondary stat gems. There are hundreds of Cata JC gems in particular. It's probably worth being explicit about the most likely and cheapest or easiest to get yourself.