With regard to soling in this game...and for the record, I played EQ1 for many years...WOW for a little over a year and have been in EQ2 now for a couple months.
I understand the question being asked, especially by people who have never played an MMO before...but the fact is, the same rules tend to apply to all of the games I listed above. I cannot speak for FF because I never ventured into that world. (But I have the knack for drawing the anger of those who play it, lol)
Every class has a strength and usually a trade-off weakness. For example, guardians defend strong but do not do as much damage as other classes. Casters can do a lot of damage but the trade off is poor armor selection meaning you die quicker. Scouts, drop defense in lieu of big stealth damage, etc.
Knowing these strengths and weaknesses will give you a general view of what can and cannot solo for you.
You can solo any class if you do it right. It can take some classes a lot longer to solo up levels than others. If you want to level as fast as possible, then go for damage dealers and live with your deaths. But the beauty of the MMO, is that you can play at whatever pace suits you. I am not about getting levels (well, sometimes I want to hit a new level for a spell or to be able to participate in a group going to someplace too high for me)...I want to enjoy the game. I have always played a healer class character (as one of several that I maintain) and truly enjoy them. I do solo a lot, and just accept that xp will be slower with my healers when i have to solo. I accept this because I have fun anyway. As I said...i am not about the end game, I am about having fun in THE game. Why would I want it to fly by so fast I miss cool stuff? Besides...you are not going to impress anyone by hitting a high level in a certain amount of time...too many are already there. Better to satisfy your own gaming needs than to ruch to the top. Just my opinion.
Pet classes are fun to solo with with becasue you are not really soloing...there are two of you. You get to tank with your pet and blast with your caster. Just as you do with a warlock or hunter in WoW...or the mage/necro in EQ1. It works well...you can move the XP bar at a decent pace, and still collect quests and adventure all over the land while doing it.
I will say that as classes have ability trade-offs...they also have group/solo trade-offs. My healer may solo slow...this is fine by me. Why? Well, because in groups, good healers are valuable. I can always get a group...and in group I have a great time as a healer. To me, the slow leveling is a good tradeoff to excellent groupability.
If you find that your character is slow in the solo game...make an alt that is a class that is good to solo. Get your main into a good guild so you can have plenty of group opportunities...solo him enough to keep him moving on quests and such...but solo your alt when you get so high level that your main has trouble finding solo work.
Oh, and another note...I find that with my healers, my ability to solo goes up and down. Usually when I get into a new zone, I cannot solo effectively there...later on I can, once the mobs are a more managable level. I group for a level or two and then I find I can wander the zone picking up quests and moving a couple of levels on my own with relative ease...then...on to a new zone and a couple of levels of tougher solo work.
I will summarize with this...classes and gameplay may differ a bit...but with respect to soloing and related, EQ1, wow and eq2 are all comparable. They are evolutions of the same thing. It was learned many years ago in the early MMO days that people needed to be able to have a good time soloing regardless of the class...and so much effort has gone into this over the years. It is a balancing act because if you make it too solo weighted, you lose the relevence of this being online...if you are going to solo only, buy a single player game, save your subscription fees and have fun! If it is too group oriented...many will not be able to invest the time and energy needed to play effectively.
Again...I am sorry to be blunt...but if you only want to solo...save yourself the monthly fees and go buy a standalone game. You are paying for the community and ability to group when it comes right down to it.
And...if you only solo, you may find that leter on you may wish to group on some of the bigger MOBs. If you do not learn to group effectively...(and this needs to be re-learned in each different game, as spells and class abilities do differ and change the rolls of the memebrs of the group)...you can be a liability to a group. Or you will not enjoy it because you do not know your role and where your strengths are. Some high level grouping experiences require every member of the group to be spot-on in their performance...truly a fun and challenging thing...but if you lack grouping skills you can find this to be miserable.
While I am rambling on about grouping...i also would recommend to everyone who does not do this already...save an alt slot to play around with other classes. It helps you to understand the roles of those classes in groups. For instance...it is important for group members to understadn the roles of the healers and crowd control members. You need to understand that the templar or warden is not just doing nothing, they are conserving mana for ehals and watching everyone;s health bar...as well as watching for adds, etc. Illusionists are mezzing mobs and you need to understadn how this works so your group does not hate you for breaking them...hehe. As a healer...I find it imperative for me to understand each class. Even down to knowing that when a wizzie blasts the mob with a certain type of spell, they often get the agro...so when i see that spell fire, I know the watch out and drop heals on the wizzie if he gets hit, until a melee can pull the agro off.
Ok...i will shut up now. Hey, I have yet to write one of my war and peace length posts on here in a few months. (beware! lol)