As folks have stated, 20 vets are more than capable. Fully enchanted and geared, they're not but a couple stat points behind 29s and while you lack a few abilities that 29s have, for most classes its not game breaking.
Generally speaking, healers are gonna suffer the least being 20 instead of 29 and you'll see the biggest difference in DPS classes, some of which are missing crucial abilities.
Just to give you an idea, I play a 20 vet resto druid and its really not hard to get myself both in top 3 damage and heals. Granted, druid is exceptionally strong right now (ESPECIALLY as solo que/pug stomper) but the level gap isnt a huge disadvantage. I think I have like a 70% overall win rate? Something like that? And Im not a particularly good player.
That said, some 20s are gonna be rough without their 29 talents. 20 Shadow priests, for instance, arent terrible but they are REALLY far behind 29 shadow priests. Things like dispersion, shadow mend, vampiric touch, etc are pretty important and you dont get them at 20. Same with say, warriors. Theyre monstrous DPS machines at 20 but they lack leap and pummel that 29s have, making them a rough choice if you're queing without a healer. Other DPS classes (like, say hunters) really shine at 20. So do a little digging and find a class that will be viable.