how are they compared to lower level chars with scaling? are the 20s still better?
Not in PvE, at least until the scaling changes drastically enough for 20s to be competitive in that aspect.
That said : 20s are perfectly capable of soloing.
As a matter of fact, besides Rogues and Mages that rely heavily on SL gems and quite often fall short even with these, any class has at least one spec that can solo.
For small tips, firstly, if you want the safest class, go pally. Prot pally has enough tools to for all situations and while it might not be the absolute best in single-target the toolkit we get is so mandatory to the class it is very unlikely we'll lose the essentials (solid AoE, healing and damage mitigation with emergency buttons).
Right now Ret and Holy can work to solo as well, be careful to your health though, especially on Ret.
For DH : Manage your heals. If you do so, it's almost Paladin-like in the other aspects. DPS spec is nowhere near as good.
Druid : Same as DH, manage your heals / procs / mitigation and you're good. Cat invisibilty is an incredible tool and you have solid damage as Bear. Resto works as well, just a little less good overall in all aspects but healing.
DK : Unholy and Frost can work, you'll have a bad time compared to Blood though.
I can't chant the praises of this spec enough. Yes you won't always have runes and the rotatipn can feel a bit clunky at first but it's not impacting your survivability and it's nothing some haste and experience can't fix. Arguably (to me it is) the best AoE available at 20. I mean look : uncapped, hard-hitting, DoT-proccing and self-healing in a single button, some more AoE in Death and Decay / Cleaving Strikes, good mitigation with Bone Armor and a shitload of HP, good ST/self-healing with Death Strike and a load of solid damage mitigation. Oh and also the Death Coil / Lichborne combo for an easy full heal.
Only downside is mobility but hey, SL speed gems (you only need 8 or 9 if memory serves).
You can easily pull 3 packs of 4 mobs, even more with SL healing gems. You can pull way more than any other class can even dream of, burn them down faster than any other class do and you still have very solid single-target.
Stop sleeping on DK, they're one of the best classes at level 20 for this xpac (let's hope it continues in Midnight).
Warrior : Same as DK for dps specs, Prot is incredible is AoE, best damage mitigation and solid heals. To me it feels like a tankier but weaker DK. Don't waste your heals and keep IP up.
Monk : Very amusing. The tank spec is easy to learn and surprisingly tanky. I remember MW somewhat working for soloing (good ST damage) and WW being a terrible time against anything hitting harder than a 5 years-old but I only ever played BM so it's prolly just me.
Outside of Tank specs, Shamans eat the best.
All specs are excellent, Enhance is slower in packs due to lacking AoE but works anyways.
Pick earth shield and the double shield talent on DPS, just go nuts on damage with Resto.
Resto is tanky, hits way harder than it should, is also quitr tanky and has the best healing spell in the game (in my opinion haha) with Riptide. This and Earth Shield are enough, blast everything at all times.
Priest : All are solid.
Shadow has damage, Disc the bulk, Holy is an in-between. All are decently tanky.
Pick depending on the difficulty of the instance. Keep PW : S up and you're good.
Warlock : make sure you don't grab too much aggro, you have insane healing, insane damage and well, even without a pet I'm sure you could still solo.
Hunter : a fair bit squishier. Keep aggro off you at ALL TIMES, use FD or Misdirection to help (Misdirection is only that useful against taunt-immune bosses). Keep your pet healing up at all times, use Exhilaration when needed and pick Rejuvinating Winds and Wilderness Medecine to help with that. Aspect of the Turtle into revive pet if he dies.
All specs are very strong. When geared I tend to prefer Surv (as you also have insane weapon options) but BM is way safer. MM works if you pick the pet talent.
Evokers : prettt sure they're strong. I don't know shit about them sorry.