For me it's was at the beginning getting what figures I wanted, or looks good to me (with my measly student allowance at the time). Then now that Im working, I set out completing my G1 - almost done still have Overlord to get. The rest of the different series I will "complete" to a certain degree only, of being complete for me. Such as:
G1 - encore, doubles (G1 insections), E-hobby stuff, old botcon, Euros - if affordable for me, yes. Otherwise just Overlord left to get.
BW - transmetal versions (whenever possible) of series cast only
TFAnimated - all released figures sans botcon stuff and recolours
Classicverse - ongoing but if not G1 based prolly not getting the fig...
G2 whatever I can get but not actively seeking - same with MPs
Unicron trilogy, TFU vr 1 - stopped and put in storage
same with bayverse and other series it's mostly stopped - getting a few figures (2-4) that I might like from the series.
NO too kiddy stuff - rpms, those pull and go stuff, none of em.
Other Liegeverse faction
Unicronians - ongoing - any figure that might be related to the dark god of TFs I will add to the collection as either an agent, a puppet, a general, etc
Army building - ongoing (Energon terrorcons, Scrapmetals, Unicron tanks)- these serve as the foot soldiers of unicronian army
Minicons - ongoing - gotta catch em all 


non official stuff - Energon cubes, 3rd party figures and accessories - if they look good for me and can fit in my liegeverse - yes(depends on price also)
Peripherals wise -
Clothing, DVDs, Comic books, books, G1 centered posters - yes
Mugs and other useful stuff (e.g. belt) - depends if I can use it, otherwise no
Lately though with only the ugly DOTM bots out there, Ive moved on to my DCU collection, so the variation and break is refreshing 
Wanted AM partner Vanguard, Myclones Dirge, G1 Victory Leo, e-hobby Dark scream ( the black version), e-hobby Magnificus
Parts- AM partner Basher-side guns, G1 Actionmaster Elite Windmill's blades[I][B]