For me, I use the original as a measuring standard when judging reissues. To me a reissue ought to be on par with the original to pass my definition of "perfect."
I consider reissues like Encore Bruticus and Devastator to be less-than-perfect because they suffer various problems that don't exist in the originals. I can forgive Bruticus more because the mould was overused during RiD... Devastator... can't think of a reason why that set has several QA issues. But for whatever reason, the fact is that they do suffer from certain defects that the originals don't have.
So for me, when I get a reissue I compare it with the original and if it's just as good then it passes as a "perfect" reissue to me. Now I think that using the originals as a minimum standard is reasonably objective measure of a reissue; reissues like God Ginrai, Sixshot, Hound, Perceptor, Skids, Ratchet, Ironhide etc., often where it's really hard to distinguish the reissues from the originals (often only the copyright stamp gives them away). I think in these cases - where the reissues are near-perfect replicas of the original toys, then these are pretty much "perfect" reissues.
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Okay, onto something more subjective: what about reissues that are different from the originals, but not in any way to do with QA? (often the changes are intentional) For me, it depends on what the difference are and whether or not I consider them to make the toy better or worse.
Examples of reissues that - QA-wise are fine, but have changes I don't like include:
+ Reissue Sixwing (WHY couldn't they have reissued him in colours approximating his G1 counterpart? Or at least made him a freakin' Autobot... *sigh*)
+ Any Hasbro reissue with elongated missiles (looks so stupid)
+ Hasbro's first reissue Soundwave. Blue Soundblaster =/= Soundwave
+ Encore Trailbreaker. The blue eye looks crap. I wish it were red like the G1 toy.
+ Any Hasbro reissue stripped of chrome (although Hasbro reissue Jazz did fix the problem with the rear roof that the previous Takara reissue had - but both reissues still have the problem of carrying over G2 Jazz's "poop-eating grin" - I've heard that Encore Jazz has a resculpted face with a more G1-accurate mouth and also with Hasbro Jazz's fixed rear roof thingy)
+ Hasbro reissue Dinobot (colours are neither accurate to the original toy or the cartoon... what's the point??)
Examples of reissues that have intentional changes that I do like and consider to improve the toy:
+ Encore Sky Lynx. The painted mouth interior and blue eyes on the "Lynx" component looks muchly cooler than G1 Sky Lynx.
+ TFC Wheelie. Still a crap toy, but the colours look nicer.
+ eHobby Astrotrain. What's not to like?
+ Star Convoy. The silvers look much nicer than the whites on the original IMHO.
+ Landcross. I like how they all have individual Autobot logos tampographed on them, and I also like the different face colours and painted eyes, although it does make them look less like the cartoon (which is more G1 toy accurate in colours). I know some people don't like how Tackle changed from vermilion to orange, but I don't mind the orange - looks more show-like too IMO (which is probably the reason why they did it).
+ Reissue G2 Battle Convoy (Laser Prime)
+ Telemocha Dinobot
+ Telemocha Blackarachnia
+ Reissue Waspinator
Examples of reissues with changes that I consider are neither here nor there (i.e. don't make the toys necessarily much better or worse than their originals):
+ Micromaster gestalts (except Sixwing). Some of the colour changes are quite different, but I don't think they look any worse. I do really like how they have individual Autobot logos tampographed on though.
+ Encore Omega Supreme's face. A big load of "meh."
+ The second Predaking reissue (i.e. "goldie"). See my reaction to Omega Supreme's face
+ Blue eyes on TFC Sideswipe
+ Blue eyes on New Year Special Convoy -- actually, I do think this looks inferior to the yellow eyes on the original and other reissues (yellow eyes tend to offset much better against the blue head wheres blue eyes - even a lighter shade - looks too bland), but it's not what I would consider bad enough to put on my "bad" list.
+ TFC Starscream. Yeah, I know lovers of show likeness may love this toy, but I don't really give a hoot about show likeness (the toy came first!)... the colours aren't bad... but I don't think they enhance the toy (not to me anyway).
+ Hasbro reissue Cheetor and Rhinox. So they glitter. Meh.
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So for me, a reissue doesn't need to be an exact replica of the original per se, but for me to consider it to be an ideal reissue then it ought to be of comparable quality to the original -- and I can accept changes that either don't detract from the toy or enhance it.