Exclusive Minifigures are ALWAYS awesome.
Parts sets, (polybags etc) are nice in that they're bonus lego, but that don't have that feel of 'exclusive' lure that a special minifig does; but they're just parts, and I can get parts elsewhere. Exclusive minifigs though? That's going to get our attention. Particularly if it's attached to a licensed line. (For instance I'm sure I'm not the only one that will get Joker's manor only for the sweet disco suits) And it's probably worth dropping a reminder about how popular previous minifig GWP, such as the Plastic Man and Martian Manhunter exclusives, ran out when they were offered.
I guess the reason is simple, most of the AFOLs I know wait for sales and discounts. Where I live there is always at least one major retailer offering such discounts. There really isn't any compelling reason for any of us to buy direct unless there is an exclusive, or an awesome GWP. But a great GWP? Yeah, you better believe I went out of my way to get a '66 batcave direct, and soley because I wanted a Mr Freeze Polybag as well (As did anyone else I know who owns that Batcave)
But, to be more specific, I'd like to see variants of the San Diego Comic-Con exclusives made available as GWP. Every year those exclusives are announced; and every year there's outcry about how unobtainable they are. Well, a lot of that is simply because those figures haven't been released at all as regular minifigures. So, how about a variant of some of the SDCC minifigs also offered as GWP? I get that Howard the Duck might not be appropriate, but how about Jean Grey? Shazam?
Or stretching a little further, minifigures we haven't seen yet, '66 Batgirl & Egghead? Lizard (the Spiderman foe)? And there's no shortage of fan fave characters from other IPs we haven't seen yet.
Or, alternatively, fun suit characters that aren't from a specific IP, but linked to a holiday such as Halloween? Or a fun suit character similar to the collectable series?
And, yeah, all of that is just a long way of saying Minifigures. That's what gets me, and the people I know, to buy straight from S@H, or a specific retailer. That's what makes a great GWP. Beautiful, exclusive minifigs.