Your detailed questions deserve a little more detailed response than "just try it".
Yes, the renderer improves slowly over time, with each release. However, it's still the bottom-of-the-bottom, imho. No fancy graphical features. There is shadowing, there is even SSAO or HDAO (but it takes a huge chunk out of your framerates). To our surprise, they took out MSAA this month, and gave us only TXAA or CMAA. Textures are still not very high-res, but surprisingly the textures look pretty smooth. Every expansion pack the world and the models get more polygons. And Blizz has done some remakes here and there (parts of the world in Cataclysm, now new models for many races). As a result, WoW has its own style of graphics. It's not Skyrim or Crysis. But once you get to play it, it isn't as bad as you'd think.
Depends on what you wanna achieve.
In vanilla WoW, the only thing to achieve was better gear. You could get blue gear by solo-play, or doing 5-10-15 man dungeons, even in pugs. That was it. To get better gear, you had to join a guild to do 40-man raiding. Or you could do PvP in BattleGrounds.
Nowadays the situation is completely different.
You can get good gear by soloing, by doing 5-mans, even in pugs, and by doing LFR raiding (Looking For Raid, an in-game pug queuing mechanism). This gear will allow to you kill all mobs in the outside world easily. However, it will not be the best gear. Best gear comes from raiding. However, raiding itself has become much easier. Raids are now flexible 10-25 people. And cross-realm. Per-group lockouts don't exists anymore. That means that guilds will probably be much more relaxed on who they recruit, and what they expect from guildmembers. I expect that even if you want to commit to fixed raid-hours where you have to show up, you can still join a guild and raid with them when you want.
Only the highest level of gear requires old-fashioned raiding. Called mythic raiding. 20-man fixed-sized groups. No cross-realm. Don't worry about that. I expect <10% of players to even attempt mythic.
Yep. See above, you can get very decent items with soloing or doing small-group content (like pugging 5-man heroics via LFD).
But more importantly, there is a lot more to do now. You can still raid or do BattleGrounds. PvP now has arenas and rated-battlegrounds. Those require a fixed team (2, 3 or 5 man for arena, 10 or 15 for RBGs). You can collect mounts. You can collect pets. You can do battle-pet fights (I'm doing those now, and to my surprise, it's more fun that I thought). You can do "achievements". There is still a huge world to explore (exploration has always been my favorite thing in games). Although not much to find, and no rewards. Just the thrill of riding around on a mount for hours and hours in new vistas. There are professions (although made easy, and not very rewarding anymore). You can play the AH to collect millions of gold (not my thing, but some people enjoy this more than the game itself). And then there are alts.
A little bit. In the past, some servers were very empty, and leveling was a very lonely experience. Nowadays, we have Cross-Realm Zones (players from multiple realms all playing in the same instance of a zone, like e.g. The Barrens). And low-pop servers got merged into medium/high-pop servers. The world feels pretty active these days.
I still advise you to find a guild. Guildies can craft stuff for you cheap (enchants, etc) if you don't have the professions. Guildchat might be a bit entertaining while leveling. And you never know, you might even do some raiding with your guild later. 5-mans are done via LFD these days. Maybe only in the first month will 5-mans require an organized group.
All classes have 3 specs (druids have 4 even). And they all have at least 1 spec for "damage dealer" (called "dps" in WoW). Leveling in dps-spec is roughly the same for all classes. Each class can have 2 "chosen" specs, of which one is active. And you can switch between them any time, any where you want.
However, when you get to endgame content (5man dungeons, raiding), there is an abundance of dps. So pure dps classes have a hard(er) time to find guilds or raid-groups. Or even pugs (when not using the LFR/LFD tools). I would suggest you pick a hybrid class. Also noteworthy, ranged dps is sought after twice as much as melee dps. Rogues (only dps, only melee) are the most useless class there is. Unfortunately I play a rogue since WoW as my main ..... If I had known, I'd play a druid or paladin.
Recommended:
Druid: tank, healing, melee dps, ranged dps.
Paladin: tank, healing, melee dps.
Shaman: healer, melee dps, ranged dps.
Priest: healer, ranged dps.
Monk: tank, healer, melee dps.
Less recommended:
DK: tank, melee dps. Need a lvl55 char to create a DK. Not popular in the community. But DKs are the best soloing class.
Warrior: tank, melee dps. Good tank. But there is an abundance of warrior players.
Mage, warlock, hunter: all ranged dps only.
Not recommended:
Rogue: melee dps only.
WoW is like heroin.
I wasted way too much time on it.
I stopped playing 5 weeks ago. Planning to cold turkey. I spent so much time reading online about the new expansion, that I thought "if I spend this much time on reading about WoW, why not play it" ? Yesterday I resubscribed. And I will play at least another month in the new expansion.
You can play the free version of WoW. But it won't give a good impression on what the game is. You can not chat with people. You can not sell/buy on the AH. You can not exchange items or gold with others. You will only see the starter areas. You will still have no idea about endgame. It might give you the wrong impressions (boring, lonely, easy game), while that might not be true later on.
Hope this helps.
Gryzemuis @ EU-Deathwing.