The good news all around is that there are people like you who feel another way than the OP, and people like me who agree with the OP, and there are lots of both of us.
That means there’s a good spread of preference between mage and lock across the community, which means they nailed it.
I don’t disagree. I do think that it’s good that different people who very much enjoy their preferred classes and specs, and I get that some people prefer mages and it is good that mages are in a good spot for those people.
Let me keep the visuals from the Diabolist tree forever, and I will never choose my (otherwise beloved) frost mage over my Demo lock ever again.
Want to see what outdated gameplay feels like? Play a DK for a bit. Just like you said, not about the numbers, just the general feel. This class feels fundamentally forgotten. We get tweaks every expansion but nothing radical.
Some classes feel much more modern than others. I find evoker, paladin, demon hunters and monk have the “freshest” of feels. Like everything connects nicely, builds flow well etc.
I haven’t played Warlock in a long time, but from the 3 mage specs, Frost was definitely the most fun one for me too.
I have to say, I love the idea of Frost DK and how badass they are - but it does feel incredibly old. I can’t quite put my finger on why exactly, but DK feels like the most outdated class to me and I have one of every class. UH also feels extremely dated to me.
If you observe the amount of iteration other classes have received vs dk (all specs) it’s quite telling. Dk has had tweaks, small changes on how components work, but there hasn’t been a single overhaul, since the Legion prepatch era. The stand on DnD to cleave is extremely outdated, especially when Mages have had their equivalent Rune Of Power removed a while ago. Standing on your special little circle to do some action is simply a relic of a bygone, slower era of the game.
Not to hijack the OP, but I find DK has clunky resource generation system, UH particularly victim of this as it’s very haste reliant.
Generally all dps have some variation of a “builder-spender” build. The fact that we have 26 distinctive dps spec/class combo that all present a different version of the aforementioned, is quite the feat. Though it is also not surprising that some are better designed than others.
I’m so glad I’m not the only one that hates the way DnD works. I try to build around it and try to avoid it where possible. That is one that stands out as being very outdated!
Another one that gets me is Frost Strike. Something about it really bothers me and I can’t quite explain it. I’m not sure if it’s the animation (for both DW and 2H) or something else, but it just feels garbage compared to Obliterate.
I have no issue with some being slightly better designed than others. I’m a warlock main and I find Demo very satisfying and not in need of desperate change. I do have an issue with some just being damn near abandoned like DK, especially when the class is a badass as DKs are.
hehe go venture on the DK forum, it’s basically one of the 3 constant reoccurring topics!
Literally just a RP resource dump, DKs issues also definitely extend in the lack of animations.
Yeah man that’s basically it, some classes just seem to get a lot less attention from the devs than others. All my guildies that play destro swear by it, so power to you for loving your class! 
Speaking mostly for myself, but generally seems to be the feel with a significant amount of people on the forums, we like the vibe, aesthetic, tmog, themes, but gameplay… Meeeeeeh! Does it have good moments? Absolutely. Is it rewarding? Not so much. Is it frustrating? Quite a bit.
i have one class and spec 1 warlock 2 aff but to all the changes it no longer is a rot and defend spec not in a long time but glad for you if a frosty makes you smile again, but for me nah, game time running out in December and going to let it run out, keeping the game but benched see you all next xpack.
Mage are just to weak
I like playing as one in dungeon or raids But for questing i hate them
They’re awful for questing. I really hated the experience, even though they do feel powerful on the offense. They just get annihilated by mobs and I absolutely detest kiting to survive.
I am not sure. I personally had no issues questing on my mage with all the additional survivability you have with the class.
I play both warlock and mage very casually and enjoy both.
Those of you enjoying demo locks; what’s your damage? (jk ofc, don’t cri)
As an “OG” demo enjoyer (as in TBC, didn’t touch the class in vanilla) 'till now- y’all know how much we’ve lost and how horrible we are compared to what we’ve BEEN?!
And to claim Frost Mages has a spammy rotation while enjoyin Demo mindless cast is bonkers. At least the frosties has some variety to their rotation while demo is one dimensional as heck it can literally be played blind, just get audio ques and remember your hotkey layout. Damn.
only time destro lock was cool was in Legion. Stayed Demo ever since.
Not as good as my enhance shamans with similar ilvl, but def. good enough I feel. Granted, things are probably different in M10+ ilvl 630+ situations, but I’m not part of those so…
While there aren’t that many buttons to click as Demolock (Shadow Bolt as builder, instant Demo Bolt as builder on procc + spreading AoE, Gul’dan as spender and trying to keep hounds and other summons up), I do enjoy the specc.
Not every dps-specc has to be like my Shaman who has more buttons to press that I can comfortably keybind, and a ton of different various proccs that I have to keep track of.
Warlock is in a great spot right now and feels very fluid and fun to play. There are always weirdos crying about specs, but Demo is not a spec that really needs any major work. It just needs a slight damage buff.
I also enjoy my Shamans right now, and they do have quite a lot going on. I wish their defensives were a little better for solo stuff but they’re generally pretty good other than button bloat.
Yeah. Burning Rush off GCD (and perhaps do less damage) is honestly the only thing I can realistically wish for with my Demolock atm.
(And more damage of course, but thats always on every class’ wishlist. 
)
They also had 2 options when TWW launched regarding Burning Rush - either make it hurt you more (like when we only had 1 point in the node during DF) or make it hurt you less (like when we had 2 points), and they got it completely wrong.
I’d say it needs a massive pet pruning, but that’s me. I dislike the zookeeper fantasy which it (and BM hunters) suffers from.
But hey! different strokes for different blokes, right? 
Yeah, I could get behind that as long as the pets that we do have left are as badass as demons can possibly be. BM Hunter just needs to make it so that you can choose the pet spec. I mean, the whole point of BM Hunter is to collect pets really.