Wow is more or less ruined by the expansion, the old world is ruined because almost noone is there anymore
I don't think the situation is quite that dire. There's a reasonable amount of Level 70 content in Azeroth, namely the Caverns of Time dungeons in Tanaris, Karazhan in Deadwind Pass, Zul'Aman in Ghostlands and the recently introduced Isle of Quel'Danas and its associated dungeons. Granted those last two don't involve any interaction with pre-BC content.
There's also been a significant amount of sub-70 content introduced in Azeroth with BC, Azuremyst/Bloodmyst Isles for Draenei, Eversong Woods and Ghostlands for Blood Elves, and a revamped Dustwallow Marsh. Blizzard also took a pass at the itemization in all sub-60 dungeons in 2.2/2.3.
Azeroth is certainly a quieter place than it used to be, but it hasn't been ignored and - at least on a high population server - there are still a reasonable number of players inhabiting the zones. The
big drop-off has been in the number of high-level players inhabiting and ganking lowbies in low-level zones, and I don't think that's a bad thing. Blackrock Mountain is one location that really does feel like a monument to past battles these days, it's odd to be able to walk through there without worrying about being Mind Controlled over the edge by a Priest hiding in the shadows or being annihilated by a 40-man raid on its way to MC.
I get the impression that you're mainly concerned with PVE, but I think the following video does a good job of waxing nostalgic about "old world" content:
The Death of PVP
instead everyone is in the expansion with its stupid storyline
I'm not overly fond of BC from a storyline perspective myself. I would've preferred it to address the Emerald Dream (where we would presumably assist Ysera against The Nightmare) or Arthas, which we're getting with WOTLK. I'd like to see Deathwing again too, he's gotta be pissed now that his closest offspring have been offed by a few dozen mortal scrubs.
I think WOTLK offers a great opportunity to rearrange the political map in WoW, namely by having Lady Sylvanas push for a march on Icecrown that Thrall isn't willing to back up, splitting the Forsaken from the Horde and taking the Sin'Dorei with them. We've seen the bond between those two races with the Deathguard presence in Ghostlands, and in how it's only with Sylvanas' approval that Thrall is convinced to allow the Sin'Dorei into the Horde. I also don't think it'd take much in the way of lore wrangling to split the Kaldorei from the Alliance either, we know that their main dispute with the Orcs these days is over their differing perceptions of Grom Hellscream and they already have significant contact with the Tauren through the Druids of the Cenarion Circle in Moonglade. It's unfortunate that there's almost no chance of Blizzard exploring something like this, players and guilds would probably take exception to being ripped apart in the manner entailed by the above.
instances that only take 25 people at most (which sucks for big guilds)
I think you'll find that most guilds have long since adjusted to the changes, some by running multiple teams. The real structural challenge has been in how Kara, as opposed to UBRS, is positioned as a true raid-level instance and the resulting difficulties in gearing a guild for 25-man content through what is largely 10-man content.
PVE Class/Spec distribution is much better in BC than vanilla WoW. Guilds actually bring Protection Paladins to raids now, and Shadow Priests.
and the stupid stupid grinding for rep put me off too
I take it you're referring to the faction requirements to gain access to Heroic mode 5-man dungeons? Those requirements were lowered to Honored some time ago, and it requires some effort to reach 70
without becoming Honored with most of the factions in question. Blizzard have also indicated that they're looking into alternate keying methods in WOTLK, guild-wide attunements and such.
As for rep grinds for gear... don't do them. I never have.