1/ enough pve content - solo, decent leveling (esp. post avatar cause not that braindead like current and no that long/annoying like before it), ow activities, plenty of instances from which even old ones were challenging (fokhm, bthm - ofc as long as they wouldn't get removed lul), low level instances worth running (see 19), raids (kn10, kn20, nexus, even wh wasn't that bad), ow events (nexus, rootstock, sun festival, halloween, beach, not sure if there was some preF2P events that got cut out and could be restored), cof
2/ enough pvp content (fwc, cs, 3v3, 1v1, duels, gvg, alliance, wintera)
3/ only +12, not shitty +15 with thousands of mwa needed and downgrades - keep in mind that clean server => no one would have thousands of mwa stacked "from previous patches" so hf farming XXXk mwa to +15 your whole set (I think during vm2 I've used less mwa on 3 chars nf/whhm gear than vm4 awakening required lol, not even saying about +13-+15 attempts) - NA players might not know it, but during VM4 there was a "GF created MWA crisis" that made the endgame unprogressable and GF needed like 1 month to sort it out while a very high number of EU players went to NA (this might be why you have stats of how mid-end VM4 was so populated in NA), also you probably have no idea how EU supply of endgame materials looked like back during vm4-vm5, it was nearly 180 different than NA so you could get a very bitter surprise when you would realise that it's GF and you should throw away all your NA memories of "how decent gear progress was during >insert patch number<" 
By high number of players I can only show stats from my guild (prime hours - 15-20 players online, after crisis ~10-15 transfered to NA, we played there for some time, later on maybe ~5 players stayed on NA, ~5 came back to EU, all other quit/took a longer break from the game)
4/ PvP gear (vm1 era) with one rerollable stat (not counting vm1) or straight +12 BR
5/ BIS gear (VM1) actually was BIS for PvE and PvP depending on the rolls
6/ no GG with vm-sets materials (I just hate this place, I take any of vm1/vm2 farm methods instead of having to do even 1 GG; and the fact GF put BoP VM materials there - just horrible); GG during VM3 was even worse - weaker gear, mandatory for materials and accessories, VIP-GG at least could be skipped (with large loss of income, but whatever)
7/ glyphs from tot/sjg, tradeable tokens, decent fun and interesting design in those places (sjg and platforms, fangspawn and barrels XD, tot and soloing all 3 bosses or afking there on mystic for profit) vs utter shithole of sabex or mcata with random glyphs from boxes (I think I've spend more time in sabex/mcata farming ONLY the new 65 glyphs than getting all 60 lvl from tot/sjg/bg boxes; I still had glyphs missing on alts after I quit ~mid vm5 cause I had enough of sabex/mcata during vm4 to even try to get them at that point)
8/ no broken classes of reaper, gunner, brawler, ninja, valkyrie - doubt that GF would bother "locking" them (so vm2/3 - reaper, vm5 - gunner, vm6 - brawler)
9/ no stupid push up skills (priest, gunner, brawler), no retarded animations so less lag (until I quitted mid vm5 I had to delete reaper/gunner animations files to be able to play in pve lol)
10/ no
- belts
- brooches
- etchings
- underwear
- any other additional gear pieces like masks, relics, helidoms or anything else that got added, less to get => faster to max gear and lul do content XD / less farming needed (sure for EQ BG it doesn't matter, but nonEQ or OW PvP hf when you aren't maxed out and you have like 50 power/endurance less than your opponent)
- gear sets for even more "power stats"
- zyrks from VG
- vrysks from VG
- random niveots from combining above from VG
- BoP bullshiet, basically from VM2 (yup) everything dropped in instances was BoP/not tradeable (esp EU VM4 >veteran< mwa, guess NA players might not know this)
- bazillions of mandatory buffs from potions, food, seals, marks, 5 types of bravery, canephora, caps, other power/endu buffs which if you don't use you can even be kicked out of parties lol (while most buffs in classic came from crafting so it wasn't that bad and in most cases top buffs like cana or bravery was for tryhards, not regular gameplay lol)
- EP points, cards, partners (whenever they got added)
- daily login bonus with power so that you can't skip login in
- fishing
10/ all reputation merchants useful (if not for zyrks, then for accessories, gear), not everything stacked in one VG
11/ crystals from bams or instances, not only from VG
12/ no crit mounts (I don't remember when they got introduced derp), only mounts with higher speed or later (~vm2) stamina/hp/mana regen (stamina for pve content, hp for BG)
13/ unlimited gathering and crafting (make ow pvp and you won't see gather bots)
14/ less forgiving content - no "death" avoid skills on reaper, gunner, brawler, no 200k HP pools (I still lol when I see some old screenshots and my archer has 53k HP on 120 stamina xD), no overhealing, no double healers in 5mans
15/ classes were harder - they had less iframes, didn't allow blant facetanking (ofc depends on content - like during vm2 even in steadfast you could facetank mchm and eat lazors like a champ, but still), mana management was a thing, no +300 crit rate bonuses and "omg I only had 99% crits on XYZ skill, mount didn't proc, tank/priest/mystic sucks cause I didn't do 12412840198109810958 dps"
16/ gathering in plenty of places (not one continent until BHS revamped gathering)
17/ extracting
18/ gear from instances had value (I could sell almost everything not tradeable on broker for gold to merchant for more than 4g per item lul)
19/ remodeler aka gear templates
20/ VM-type of gear required WAY LESS materials (overall, which corresponds with the amount of clears you needed of instances to get enough materials for your gear) - compare the amount (just the amount) of materials needed for vm1 or even vm2 (I think vm2 was easier cause drops where better) vs vm3/vm4/vm5 - in terms of nonEQ PvP I would say it was even worse cause the amount of reputation credits skyrocketed with vm3 or so
21/ Good enough gear was BoE, tradeable so running instances gave profit even if "BoP item" didn't drop [vm1]
22/ no IoD BAM farms (whenever it got added)
23/ COF (even if it was bugged)
24/ alliance system or the vanarch system (crusades were interesting, dunno why they deleted them tbh) - cannot say more about alliance, I don't remember how alliance looked during vm1 and then during vm2
25/ guild levels and doing blue quests for awards (when I logged during stormcry patch I still had like 3k of those awards XD)
26/ personal guild banks with 4 slots
27/ no wide available proxy or dps meter that resulted in less elitism (imo)
28/ overall forming parties was faster and easier (more players), players were also more friendly (no idea how its now, but my vm1 guildmates taught me sjg, ac, mc, kn, etc, while during vm2 we still made practice runs and didn't care of the effect, even mchm) and didn't cry that much or disband after 1st wipe blaming everyone left and right (imo)
29/ KG was fun (different approach to kill the boss)
30/ rookie instance buffs (free CB, more dmg, 1 free ress, rookie loot bonus) - this gave you reason to run with unskilled players to help them and in return you were getting more loot (with a guildmate we were exchanging kn5 runs on alts for this bonus)
Ofc vm1/vm2 had plenty of flaws, but I personally still think it was way better than anything past reaper.
PS the gear part surprises me the most tbh - the amount of materials on the server would be ZERO at day0, there would not be players that skipped "previous" content and have 50k mwa, 5k mes or 20kk gold stacked. So I can't really put into any logical argument why anyone would want past reaper updates that started to add MORE and MORE stuff to grind/farm when GF barely manages to provide decent sources of any items.
As for a "skip leveling" thing - I don't say no, it could be a decent thing in TERA Shop if did correctly. Only thing I find bad with this is - you get insta 60 and have zero resources. Cannot buy skills, don't have any leveling gear, can't get glyphs, charms, potions, crystal. So yea. And if "skip leveling" box would give some of that stuff - P2W from day0. Could be okayish maybe after like 1 month of the server being online or so when there would already be 60 lvls running around.
Or this could be a reward for leveling your character to 60 so that you can get your alt insta 60 for free (with alt "no resources" isn't a problem as you have resources or ways to get them on your second max level character).
This could also be tradeable, I would 100% buy it from broker instead of having to level up a mystic for pvp
(did it one time during vm1, that's enough)