Sorcerer was great in Beta and since then has been nerfed along with Bloodmage, and Disciple. I know a few others have been hit and I haven't been following it closely.
Brad believes in forced grouping and some classes will still be able to solo better than others. That solo ability will be limited and in some cases, as you have discovered, will not be possible. Some quests and powerful creatures may very well demand a group and you are SOL if you can't get one.
I can't currently give you very accurate opinions or advice on the latest round of nerfs because I have stepped back myself to let things settle. In certain cases some classes are better than others. Wolf totem shaman use to be great because they had decent melee ability and could heal themselves. Rogues are good for working into hostile areas because you can often sneak by aggressive creatures. Thing is, once you engage, you may gain aggro from wandering creatures and those nearby. You are not a tank to boot, so fighting a creature one on one is more difficult.
Some creatures are indeed "linked" and no matter what you do more than one will come. If you have tried pulling them from different directions or pulling the outer most creature and they always all come they are likely linked and there is nothing you can do beyond grouping. In MMOs some creatures are grouped. Period.
My advice is to read up as much as you can and perhaps ask for help on the official forums. I am out of touch with what has changed very recently, and am waiting for the game to "settle".
I definitely suggest you try a few different classes before you lock into one.
I am getting really tired of MMOs because most of them don't know how to do Rogues well, and I always want to play at least one. Missions are; kill, kill, kill. Rogues ought to have meaningful
XP that comes from something other than fighting. There's no trap setting, pocket picking, breaking and entering or item stealing. Heck I would even accept urban recon missions. Rogue missions ought to be about avoiding detection as well.
If you really
want to solo in an MMO get EQ 2 or WoW. If you do EQ 2; try a Conjurer or Necromancer. Illusionists are very good too if you know what you are doing. If you want to go more melee, a Shadow Knight. If you want to play a "solo" healer, a Fury. XP will be slow in EQ 2 post L39 and you still will be able to solo.
Oh yes, and before someone comes along and asks, "Dude, why do you want to solo in an MMO?", I will address it. I want the option
to solo, not to always solo because sometimes you can not get together a meaningful group or
the group you could get into is replete with tools that would make you miserable. So, not being able to form a group do to lacking players or classes or said players being idiots. Grouping with some folks will actually lead to no fun, loss of XP and damaged gear. It's lose|lose. If
I am having a great time and die plus take an XP hit, I don't care.