There are some free games that I find worthwhile. However, I think really answering your question is impossible. We can all say which are the games we think worthwhile. But, you want an introduction to the genre to see what it can offer. It is an odd request to answer. I will suggest a couple of games that are very different from each other and also point out some things you will want to think about as you develop your 'taste' for mmorpgs. (Assuming that you are going to continue to apply as much thought to this as your initial post suggests.)
First, to develop some basic themes, you should read the "Common Features" section of this article
Character progression is one of the big 'things' that people that enjoy mmorpgs desire. If you find that you do not enjoy thinking about developing your character, you may find the entire genre to be 'unfun'. If collecting XP (experience points) is enjoyable, then you are well on your way. This leads to a question about your enjoyment / tolerance of 'grinding', as described by the linked article. Some games do a better job of making 'grinding' less obvious / more enjoyable. As far as I can see all games that do not simply allow quick leveling have some kind of grind. So, if you just want to hit max level games with a 'grind' will be less enjoyable. If you enjoy the journey, then grinding can be more or less enjoyable.
On the issue of teamwork...some games focus on fostering player interaction and other games focus on fostering solo play. You will likely find that this distinction is important to you.
On the issue of roleplaying...if you come to this with a background in traditional pen and paper RPGs, then you may have a desire to try out roleplaying. Most players of mmorpgs do not roleplay. You will have to search out a community of roleplayers, which exist in every mmorpg that I have searched for them in.
On the issue of pvp (player vs player combat)...you may find that pvp is your thing as you come from fps. If so, you will want games that support interesting pvp options.
The f2p (free to play) vs p2p (pay to play) distinction is also one that you will want to explore. Depending on your mindset the various ways that f2p mmorpgs generate income may bother you or not. The other difference is in the casualness of players that a f2p game allows. No cost allows for minimal commitment. The only way to know what you think is to try.
It should not go without saying that there is a big difference between computer /console rpgs and mmorpgs. mmorpgs are generally much weaker in the story / plot department. This is very disappointing to some, but it will depend on how much your enjoyment of games is plot driven.
For a basic introduction I would consider Rappelz. It is a fairly enjoyable free mmorpg. It allows for both solo and party play. It has basic character progression focused mainly on skill choices. It has a pet system which is a growing element to mmorpgs and probably should be added to the linked article at some point. It allows for pvp in arena type settings. One of the problems with this suggestion is that it had poor documentation when I was playing it. I would hope that has been addressed by now.
After an introduction with Rappelz you might try Ryzom. (Which was still free last I checked.) This is a very different game from Rappelz. Everything from the economy to crafting to pvp is different. pvp can come through guild wars to take an outpost from another guild. Having an outpost gives benefits to the guild. Once you have played those two games then you will be well on your way to figuring out the details of your preferences.
I have spent some time playing both those games. I am not currently actively playing them, but they held my attention for a time and I think they fit your desire for basic. There are a number of games which seemed focused on children and, therefore, may be more basic, but I do not know.
I hope you have fun figuring out what you think of this mmorpg business. I have!