Assuming you're saying you have the option to just make a level 90, I assume you have a character boost, just go ahead and make the 90. Freshly made 90 boosts only start out with a few abilities and do a bunch of quick quests to unlock the rest now. Besides, with the ability pruning, there is no issue with having too many skills on your bars.
Take it from someone that has levelled 10 classes to 90 and boosted 1 with my free boost. Solo leveling in the old content is dull. Dull, dull, dull. If you want to jump back into the game and experience the most contemporary and modern content, just go ahead an make a 90 and join all the other players new and old in the latest expansion. Besides, level 90-100 aren't over in a day. You have plenty of time to learn your class in that window.
By the time you get to 90 leveling the old fashioned way, you'll miss out on the best part of any expansion. The very beginning, where people are out in the world and active, giving the world that vibrant and living feeling.
Anyway, to directly answer the question, any class can solo effectively these days. DPS specs or tank specs are very viable for leveling solo. Healer and tank specs are great for dungeon spam.
And as for your bonus question, allow me to quote myself from another recent thread.
The WoW community seems healthier than ever on Horde side, and worse than ever on Alliance side. Horde side mostly has progression focused players that know how to act mature, no matter their age, and put in the effort to get things done or get out of your way so you can get things done. The very worst I've seen since I started playing Horde was some anal spam in trade chat and the occasional asshole here and there.
Alliance has this incredible divide between much older and extremely young players that creates this atmosphere of clash between its own players. I tried raiding with Alliance on an alt for some time, the players were lazy, uninvested in the game and prone to rage quitting and arguing with each other. In PvP, Alli has a very XBox Live feel to the community, where I see the most profane shit in chat whether you're winning or losing. While I experienced a bad player from time to time on Horde, on Alliance I experience them daily and in large numbers.
As a note, I've been playing Horde and Alliance for about 10 years now and my experience of the factions come from US-Thrall Horde and US-Frostmourne Alliance.