By definition I want anything that's Lego. What you need to learn is distinguishing between "that would be neat to have" and "oh my god I must have it or my life is incomplete". Basically I sort things in three categories:
* Won't bother unless it's basically for free (like, I would buy anything at 90% off for parts or gifts or even investment but that doesn't happen)
* Will wait for a sizable discount, around 40% or so. If I don't end up getting it, no big deal.
* Absolutely want, and will get ASAP at retail price since it's not worth waiting months for maybe saving 10% or so, or risk missing out if it's a super hot item.
I have three categories as well, but they are a bit different:
Level 3 - 50% off. Any LEGO system set that is 50% off falls into this category, regardless of theme. I can't walk away from deals like that unless I already have the set. This is why I have a bunch of unopened Ninjago sets.
Level 2 - I'm sleepy and I accidentally bought a LEGO set. Typically these are the $15 - $100 sets in the Star Wars and Super Heroes themes. I usually end up getting all of them without really planning on it. I'll often buy the smaller sets from various one off themes in this fashion as well.
Level 1 - I need it, and I will stress out about it until I buy it. These are typically the $200+ sets like Brick Bank and the UCS Tie Fighter.
There's also another nebulous category that I can't wrap my mind around, the sets I think I "should" have but I'm not really sold on. Sandcrawler, Ewok Village and GB HQ sets are like this. I'm not crazy about the first two but I know I'd enjoy them. I think I only want the latter because it's the third biggest set ever...which is a dumb reason to want something.
Oh! I forgot to post this over the weekend. Spotted these sets in an actual retail store in my town. Gross.