Sorry I haven't chimed in sooner!
First, if a major change occurs due to an error in manufacturing (text especially) that first one is rare and becuase it goes through changes (error, sticker, printed), it becomes out of print and off the pegs quickly. Some get attention and attract enough interest to create a demand that increases the value. I think over time, these will still hold their value and probably increase, but I am not an oracle or very good with judging the market...
The most difficult variations/errors I own are...
#1-16 with background inserts
Clone Trooper - missing black paint on visor
Clone Trooper - missing dots on photo
Luminara Unduli - cloak text visible/no bronze paint on necklace
Dexter w/insert - pipe text visible
Padme (Arena) with background insert and mole
Padme Amidala (Coruscant Attack) - w/correction sticker
Super Battle Droid - with correction sticker
both R2-D2 leg variations
Anyway, I have quite a few. The one's I covet are the original Super Battle Droid with the error printed on the back, all Imperial Officer and Rebel Soldier head sculpts, Kit Fisto with lightsaber packaged in bubble (not in hand) and Jango or Boba Fett without the black visor paint.
As for the questions...
Luminara is worth it I think. It's a hard find and a nice piece to have. I do not collect for value, I try and collect figures I like and figures that other traders want so I can get what I want. Any of the original text variaitons are good to have because there is always going to be the collector that is out looking for these things. I had the error text Super Battle Droid and traded it and recently tried to get it from someone and they were more interested in trading POTF2 ss/lt figures than that variaiton. I also am trading my Clone w/o black visor paint for a considerable amount of good figures. It's hard to let go of, but if the price is right...
When the stickers start coming into play (i.e. 'Force Action', or 'Slashing Saber Action' it can get pretty out-of-control). My rule of thumb is if there is a change to the figure packaging (text or photo) or a change in the actual figure (or error that is reported enough to warrant a variation tag) it's worth grabbing. Some (like Han Solo with white or silver belt lever) will never really be that big a deal because it is difficult to gauge which is the more rare of the 2.
I am very interested in any images I can get as verification of the existence of any and all variations and/or errors that are not on our lists. If you find one, send it in please! I am working on starting some forum threads with resources from our checklists and those that have contributed. Thanks and good hunting!