MMOs are social games by their very definition. While their game play is generally somewhat engaging and robust, most players would agree that without the interaction of other people surrounding you, both in group and out of group, you'll probably get a much more rewarding experience playing a single player RPG. And that's where there's yet another hurdle for a trial account user to surpass.
Only a few years ago, trial accounts were actually relatively unfettered. There were still a few restrictions, such as a max level, but you could still freely send tells, join public chat channels, trade, and so on. This was before the gold sellers really started taking off. As they got more and more ruthless, gold sellers began using trial accounts as mules and ad bots. As a result, the full list of Trial account restrictions has gotten longer and longer.
Spamming whispers and public channels with ads removed the ability to join public channels or to whisper anyone whose friends list you were not on. Creating raids and sending random people invites in order to spam them lead to a loss of the ability to start groups or group with people above level 20. Using trial accounts as mules to move massive amounts of gold lead to trial accounts losing the ability to trade or use mail or the auction house, and to a limit of 10 gold earned at any time. They also lost the ability to create or join guilds, also for spamming reasons.
In other words, most of the social aspects of the game are almost completely cut off from the inquiring trial account user. In theory, you can still use the /say channel to communicate with others, but very few people use say these days. If you want to group up with another trial account user to do a few quests or a dungeon, you can't. You pretty much have to sit around and hope someone with a full account will randomly decide to communicate with you. Essentially, you're playing a single player game.
Of course, the problem is that all of the issues that caused Blizzard to place further restrictions on trial accounts were real, and the restrictions have certainly reduced gold farmer spam, if nothing else. But chances are they've also cost Blizzard a few customers, because trial accounts no longer give you a taste of the true game that is WoW unless you already have friends playing who can friend you and get you past some of the restrictions of the trial accounts.
This may be in the end why Blizzard's trying so hard to revamp the old world. Since it doesn't seem possible to have unrestricted trial accounts that take full advantage of the social aspects of WoW, the best way to hook new players may be to make sure the first 20 levels of WoW or so are essentially a high quality single player game.