I've not had anything that I've ever fielded rendered unusable by a rule change since third edition. The change from second to third edition was so significant that it was bound to invalidate some options, but otherwise I've fielded more or less the same units with the same weapons since third edition. I don't optimise my wargear/units for particular editions, I pretty much use them as is. If that means some weapons are less effective from one edition to the next, that's just the way it is. This is why I prefer all round lists which are not excessively tailored or optimised.
I don't optimize my armies for maximum competitive power either, but I do generally like to have my units optimized, or at least not intentionally crap, since I hate feeling like I'm intentionally wasting points. For example, with the new Inquisitorial Acolytes, I will not be using any of them with Laspistols. Bolt Pistols are the same cost, and unambiguously better for zero downside. Also, I haven't been playing since 3rd, I've been playing since 5th, and I tend to get new models and units regularly - I played only Orks in 5th, added Space Wolves, then Space Marines, then Inquisition, then Sisters of Battle in 6th, then added Grey Knights, Deathwatch, and a few other minor bits here and there in 7th. (Assassins, Astra Telepathica, etc.) I built those with whatever seemed coolest at the time.
However, most of my invalidated units are simply combinations that I think were cool: I took an Inquisitor with two Daemonblades, because the idea of him carrying around the souls of Daemons in order to achieve the maximum human potential of power is really cool to me. On the board, it was a crapshoot - Sometimes incredibly powerful, sometimes worthless - But it was always fun to play.
All of my models that I built to have WYSIWYG with relics (Most pertinently, my Wolf Lord who I gave a bonesword, so that he could really weild the Krakenbone Sword, and my Librarian who I gave a Thunder Hammer from the Iron Hands upgrade pack so that he could have the Mindforge Stage), are not really useable. My Wolf Lord can, I guess, have a Relic Blade? But that Librarian can't be a Librarian anymore. My Cataphractii Captain with a Storm Shield is bust. Sororitas Command Squads don't exist anymore, Psycollum doesn't exist anymore, and Condemner Boltguns barely do anything anymore, so I have nothing to do with the five Condemner Boltguns that I have five Sisters all armed with. Most of my Deathwatch are bust (Though incidentally, they randomly and suddenly allow Deathwatch Veterans to take Bolt Pistols, which they don't have parts for and require kitbashing because that didn't exist in 7th).
Usually, my list building involved a sort of reverse optimization: I would pick a list or army style that I want to try, then retroactively figure out how to make that list good. I've played melee-centric Adepta Sororitas, Space Marine Parking Lots, pure Inquisition, pretty much anything I could think of that sounded fun. Sometimes that would lead me to stumble upon something genuinely OP, but OP lists are incredibly boring to play, so those lists usually get shelved after a single use.
GW has always taken this approach with their rules, as you alluded to above. The only difference this time is that any changes will probably be made more quickly. If there are combinations that you are convinced that they may change, hold off constructing the models until there has been a review. I doubt that you would have to wait that long.
My Inquisition models were legal since the beginning of 6th edition. Are you saying that I should have held off for that long? Sometimes those changes can be predicted (For example, it's not too surprising that they changed Deathwatch a few months after the codex came out,) but other ones are a complete, unfair crapshoot. (How in the heck am I supposed to predict that an entire unit is going to be axed from the SoB codex?)
Also: I shouldn't have to wait six months or a year to find out if GW is going to pull out the rug on me before I start building my kits. If they can't decide what they want to allow before they release it, they shouldn't punish players for that.
Randomisation is tedious, I agree and I also prefer heterogeneity to homogeneity, however, I would have to be persuaded that this edition involved too much homogenisation. This is not to say that I think that eighth is somehow a silver bullet to all the problems with GW's rules. I do not believe this for one minute, but I do think that, in a number of respects, it is a step in the right direction.
I don't have exact data on it, but I think that a lot of weapon profiles across armies definitely are a lot similar. If you look for the number of weapons that used to have different rules, and are now S8 AP-4 Bonus Damage at Half Range, or S9 AP-3 D6 damage, I think you'll find that a lot more weapons have a lot in common. Also, the loss of AP5 and AP6 equivalents means that most anti-infantry weapons are a lot more similar in effect, as is the fact that all weapons are useable on all things - Since S3 can now hurt tanks just as well as S4, instead of S3 being useless against tanks, but S4 being sometimes capable of glancing tanks from the rear.
Why should I take a Vindicator over a Predator now? Or visa versa? You get +1 Strength and D3/D6 shots instead of 4. They do the same battlefield role. What's to get excited about? A slightly different type of rolling that does the same thing? I may as well just go with whichever gives the slightly better cost/damage ratio and call it good.
Combi-Weapons are just regular Special Weapons plus Bolter Shots. Flamers fill almost the exact same niche as Storm Bolters, getting almost the same number of average hits and doing the same damage since it no longer ignores cover. Plasma Cannons actually do the same amount of average damage as Plasma Guns when within 12", since D3 shots means 2 shots and that's what you get from a Rapid Fire weapon anyways. The Command Land Raider now just gives the same benefits as a Space Marine Captain.
Captains give re-rolls on 1s to hit. Chaplains give re-rolls on hits in Close Combat.
A regular Captain is as accurate as Cypher now, because 2+ with Rerolls on 1s can't get any better than it is.
Ork Deep Strike is equally as accurate as Space Marine Deep Strike and Eldar Deep Strike.
Chapter Tactics and Marks of Chaos have been axed. (And no, 'Just wait for codices' is not a good counter-argument. There's no reason why they couldn't have included them in half a page in their respective books, and having to wait months or more to get content back to the point it was pre-update is crappy.) A unit of Chaos Space Marines now plays identically to a unit of Space Marines in almost all circumstances.
Acts of Faith give the same buffs to every unit, and one of those buffs - Recovering wounds - Is actually the same as an ability provided by other units. (Actually, Hospitallers are just less versatile versions of Imagifiers, since they do the same thing as an act of faith: On a 4+, recover wounds for a nearby unit.)
Also, this is a minor point, but Ministorum Priests only give one buff, and it's only useful to a tiny number of units that could potentially take it, and Astra Telepathica really only help IG.
I could keep going, too. For a really, really long time. But I think this helps make the point.
The comments pertaining to overpowered combinations are issues that I find unpersuasive. Having played Rogue Trader and second edition, most of the balance issues since then have been far less serious. There are likely to be balance issues, but this has been true for all editions. I think that it's far too early to complain about balance in eighth when there's so little data to analyse at this juncture.
I don't mind imbalance, but I do mind imbalance in this particular context: If the whole point of 8th edition was to flatten out the balance, and that's why they had to sacrifice the unique abilities, weapons, and rules that all the different armies and units have, that's fine. But when they remove all of those things, and the game is still increadibly breakable at a glance, that's a major problem. (And from all of the data I have: The deployment rules are crap, and first turn leads to an almost guaranteed win. I've played four games, and this has been true from all four games, plus all of the games I've heard about from other players, with one exception: A killpoints game between pure Imperial Knights and Orks, in which first turn did not matter.) This will require more time to confirm for certain, but it seems pretty clear already that the rules for deployment and first turn are incredibly imbalanced and unfair.
It feels like, instead of trying to balance the game, they just tried to remove all of the things people were complaining about instead, without realizing that their new fixes caused new breaks.
I get the impression that you play a lot on a highly competitive tournament scene. That has never been my cup of tea. I'm a casual player. I suspect that this is the main reason why we have a number of divergent views, although we do agree on some points.
Not really. I play with one CSM friend at his house, and then a group of friends at an LGS, and another group of friends on different days at the same LGS. If I were a WAAC player, I'd be happy about how cheesy the new edition appears to be: More tournaments for me to break, after all!
I'm disappointed because I like playing with all my different toys. I like putting down my big models and smashing them together while making dinosaur noises. (Proverbially*.) It's a game, I enjoy playing with it, and by stripping out a ton of options and ways to play (While ironically adding two game modes that are unplayable if you want a remotely fair game,) GW has removed many of my favorite things.