Then you'd be happy to hear that Runes of Magic requires neither purchase nor grinding :)
Also, the items for sale there are convenience items. They are both truly useful and not required to complete the game. You should really try out the game before forming an opinion, chances are you'd even like it.
If you want a more difficult WoW, you probably need to go back to old-school games like Shards of Dalaya. It needs good coordination, strategy, planning and all that, but the downside is that you are forced to find a group (as early as level 6) because you simply won't survive without a balanced group of roles. It all has advantages and disadvantages, I don't think they'll ever succeed at making a game design that is easy but challenging, group-friendly but solo-friendly etc. There are many opposites that players want to have BOTH of, and that won't work.
I don't see the potential for impulse buying as a problem at all. You play the game for a week, you think "bah, I'd like to travel faster", so you buy a few transport runes or rent a horse. Horses are cheap to rent with pure in-game currency too, by the way. After a week you find out that you won't have time to play much in the next three weeks, so you won't be spending any money on the game either. With a subscription-based game, you'd still be paying money.
For people like me with our three hours a week game time, free RMT-based games are perfect and subscription-based games would be a waste of money, since I'm paying for time I can't use anyhow.
But seriously, download RoM, play it for a month. Then form an opinion. I see many people who are so opposed to the idea of trying an RMT game that they keep repeating mantras they've heard from some people who tried some OTHER RMT-based game -- every game is different, and you have to try them first-hand to really have an opinion that counts.