WoW is surprisingly very casual friendly these days. I've played since beta and it has never been as easy to play casually as it is now. The last time I played was just last month and with no guild, no friends on the server, just me alone, I still managed to pug almost everything. The only content I didn't get to experience was the latter half of ICC, mainly because my gear when I restarted was barely Naxx capable and I was more interested in playing all my alts instead of focusing on one.
Personally, I've devolved from hardcore vanilla raider into the hardcore casual, so I simply subscribe a month or two at a time whenever I feel the urge to see the new stuff. With seven max level characters, it's very easy to just pick one or a few and gear up whichever I feel like playing. The best improvement over the years in my opinion has been the talent trees really making each class play three different ways, so if you're bored with one play style, you can either respec or switch to an alt.
Oh, and gouging out some more gold from the auction house took some time as well. I dropped over 30k gold buying mats for boe gear. This time around, Blizzard added 2 patterns per raid instance per armor type/spec so if you have the gold, you can get two icc25 level pieces, two toc25 pieces, and 2 ulduar 25 pieces without setting foot in instances. However, I would recommend the toc25 only since those are bet bang for buck and heroics drop badges for offset/tier gear.
Pugging dungeons is much easier with cross-server queues, though you pay for it with the jack asses who go afk or jerk around. Even heroics are relatively easy since so many people have icc gear. If you run into a group with non-icc gear, it's even a bit refreshing to actually have to pay attention. Queue times for the ruin battlegroup are about 10-20 min for dps, 1-10 min for healers and 1-5 min for tanks, usually very fast during peak hours.
Depending on your server, you can even pug all the raids up to icc25. Heroic modes are probably out of the question for the most part unless you get a lucky pick.
However, for the most part, you can easily pug the first wing of icc while brain dead. I last played on Lothar Alliance and while the raid progression is somewhere in the back, finishing the newest raid instances weeks or even a couple months after world firsts, the server has always finished, so pugging the latest isntance is simply a matter of time. Some servers are so dead or devoid of ability they barely progress through raids during the entire expansion. Others, as mentioned previously in this thread, may have very good hardcore raid progression, but casuals are left in the cold.
If you're feeling the itch, I wouldn't really wait for Cataclysm. Blizzard would have to rush a few things to make a Christmas deadline. Whether they do, only the execs know, but either way, like both of the other expansions, there will be a pre-patch on the live servers to update everything in preparation for the live push. All talent trees are going to be overhauled to an extent, though never as much as they say they want to do. All armor stats are being overhauled again, consolidating and shifting gem colors to reflect changes. It would probably be better for casuals coming back to the game for a short while to see missed content while the game is still using the system and mechanics they're used to. When I re-subscribed last month after being away for almost a year, I only had to figure out new talent specs for a couple of my alts before hopping into a dungeon queue. With cataclysm, it's much more than just talents moving around the tree or modified slightly and it's affecting all classes and specs, not just a few.
