You can solo pretty much every quest in runescape, except maybe 2 that needs another player :P
The storyline is unfinished tho, you get to do alot of quests that somehow will relate to each other, leaving other quest for you to wonder if they connect somehow in the future

You can do a good chunk of Star Trek Online solo and you never have to talk to other people though it can get pretty fuckin hard and you have to play smart & careful when you have to take down a dozen dominion cruisers alone. I never finished it though, I hit a bug that affected a skill that I used almost constantly and it was enough to turn me off the game. Must see if they've fixed it one day (any other STO players here? Have they fixed the high yeild torpedo & beam overlload skills?)

That game is good for loners even for group work since it auto-creates teams and you can leave them whenever you want. In the large scale fleet actions, I just took to treating the other players as argumentative AIs and using them as cannon fodder whilst I focussed on the mission, worked pretty well.

STO is one of the three MMOs I have tried now, the other two being Guild Wars and WoW. I got STO, limited edition even, because I am such a Star Trek nerd. I agree that it is solo-friendly, and even when it did put me into a fleet battle I just ignored everyone else and it was fine.

Unfortunately I found the quests and exploration both to be lacking. Maybe later in the game there is more cool stuff to do, but the first 10 hours or so was just endless repetitive space battles with bland text as a intro.

No way, Lotro is probably even more unforgiving as a solo player than WoW. There's no Paladin or Druid class types that can solo instances easily. I'm a max level Loremaster myself back before they released the exppacks. And visited most of the 5 player dungeons. Lotro presents it's quests alot better than WoW though. Main storylines run as Chapters and so do your instances. It feels more mature, so is the community well, at least it was back then when I had to pay sub fees. The Shire in LOTRO is still the most peaceful and scenic place I've seen in any MMO.

Don't bother with Maple Story. It's a complete waste of time. It's just a horrible grindfest and $$ sink. There's NO storyline. You just go around staying in one place and grinding mobs till you level up. That's it for Maple Story, seriously. And Maple Story is idiotic. I'm a Level 135 Mage, it takes me 4 hours to gain 4%-6% of exp points out of 100% to level up, yes 4 hours seriously. Everytime I die I lose 5% exp points which essentially wastes the 4 hours I grinded for. To prevent that from happening I need to spend $1 to buy a card. And everytime I die it's $1 gone. Maple Story's version of the AH is the Free Market. You CAN'T search the Free Market, you have to buy an item that costs 50 cents to search the Free Market for ONE item otherwise you need 1 hour to search through every single store.
You can buy double xps and double $$ drops, but that costs me about $20-$30 a month. To Yell in Maple Story, you need to buy a Megaphone which costs $3 a yell.
Seriously though, I don't believe that WoW is the MMO that earns the most money. Collectively, I believe Maple Story is the one that nets the most cash globally. It was after Maple Story which I played for about 6-7 months that I became incredibly anti-microtransactions. I got a in-game friend who spent $3000 on the Maple Story jackpot to try get a powerful item but didn't. I alone spent enough money on Maple Story for 6 months to last WoW for 2 years worth of subs.

I actually find RuneScape's quests to be pretty good. Most of them have semi-decent plots (some have pretty good ones) and revolve around stories and puzzles rather than Fed Ex quests and the standard "Go kill X number of Y monsters and I'll give you Z for a reward". Runescape mostly suffers in its shallow combat and tedious mouse-click gameplay, as opposed to something more action and skill-based like World of Warcraft.

Nonetheless, I go back to Runescape once or twice a year for a break from everything else. It's easy to get into, very solo-able and has decent casual gameplay. It's also the kind of game that's easy to play while watching TV or reading. It's not particularly engaging, but still some pretty good fun. The quests are definitely a high point.

Jagex is actually working hard on improving the standards of the game. Recently it's been suffering a great deal from its history as an indie MMORPG (poor initial design and gameplay decisions, etc), but recent developer blogs indicate a number of elements are being overhauled. Recent examples of this are the hit point changes and the removal of some of the poor-quality quests that have been in the game since the beginning.

It used to be, Lotro's been greatly nerfed to a shell of what it used to be though. Sure some of the classes still kill stuff slower than others, but there've been many zone revamps (Angmar, LL, all the starter zones) and mechanic changes (much smaller aggro range than before, rez-where-you-die, the crazy-solo warden class, and others) that've made the game a lot more solo-friendly, especially now that they're trying to draw the casual crowd in by making everything a lot easier, and (slowly) making all the dungeons scalable.

They're just doing what worked so well in DDO. Don't think of it as casual, think of it as not insane, hard core, BS. There are still plenty of hard core games out there. LotRO always should have been a "softcore" MMO (teehee), just to draw in fans of the universe but not big time MMO players.