A wonderful thread if you ask me. This is something I have spent a lot of time thinking about some years ago. For me it all began with my first MMO which was good old Ultima Online and I played that for straight 10 years (sounds weirder now that I wrote it than it felt). That alone might tell you how much it amazed me and stunned me for many years - but as any MMO even this amazement has faded, without switching the game. So it cannot have something to do with that or with a specific game "missing" things, at least not from my perspective since I never switched it and the feeling still disappeared. One might ponder if it was all because I was so used to it - but that was not the case either, since after those 10 years I did switch a lot and was in many many betas (Everquest 2 among them btw) - but none has stunned me or had an equally charming appeal to me. Partially that was because I was not willing to sacrifice the huge amount of roleplay we had going in UO, part of that certainly can never be explained properly and some parts of it I have discovered after pondering, I think.
What really makes a game shine and stand out, amaze you and drag you in - I for one had a good amount of that reappearing when I began to play SW:TOR with my spouse. That amazed me and I pondered and thought about why it happened here but didn't happen in EQ2, in Eve Online, in DaOC, in Neocron 2 or any other MMO since Ultima Online. Then I started to compare those and eventhough SW:TOR is much much different from UO it does have certain core features that link both, and first of all that is: having an interesting lore and universe. I remember that back in UO I used to frequent a website about it and even printed out the lore stuff from there as well as some newbie guides - and I found myself archiving lots of lore content and spending many many hours on Wookieepedia when I began with SW:TOR because I was sincerely interested in the lore (I always had a soft spot for the original SW movies but I never was a die hard fan... until sw:tor).
A second link certainly is the feeling of being part of something glorious, something very new and shining - as for example sw:tors countless cut-scenes and dialogues that never fail to amaze me, right now I am deeply in love with the designs for the Grey Secant (Tron, anyone?) and this feeling has not faded ever since, I still consider sw:tor to be the top of the list when it comes to quest narration and I can understand why a MMO does not offer tons of different solutions to quests (it would be an insane workload) - rather than wasting my energy on lamenting about that, I do enjoy the hilarious one-liners for example my Bounty Hunter drops ("I maybe negotiated with his face a little."). That one even made it into Real-Life communication with friends and guildmates here.
Third comes the last link I have discovered that might have something to do with the affect - and that is the game's level of thought. To explain what I mean with this - I do mean the way things are designed and the reason behind that. See.. as I have been in many betas, many MMOs and such I did see tons of bad decisions. When I began SW:TOR I was repeatedly being heard saying things like "Wow, they thought about that? That's quite amazing" and "So.. they blocked this since it could lead to that exploit? Cool thing" - and that is just part of what I thought. Sure, there were and still are bugs in SW:TOR but I dont know any MMO without bugs, nor do I think I ever will. The reason why this amazes me and makes me happy and feel "magic" is because of the way the Devs approach the community - they do it on par but they also do NOT feed the trolls, they don't say "Oh sure we will quickly do as you asked us to do! Sorry, dear customer! Will be fixed right now!" but instead they evaluate community reports as much as their own data. Trust me, fellow readers, that is so incredibly rare and brave that it amazes me. It is the same reason why the Ultima Online community never had many problems with whinery and players "demanding" things in the long run - because they knew that was pointless. Solutions were found where they were needed and if someone was being ganked all day long and robbed clean of all items (that was possible in UO) ...then it was his bad personal luck and his fault for carrying around all the valueables he had. No use to complain. That is essentially how it is here and I really appreciate this. The absolutely worst community-to-staff communication I have ever ever ever witnessed was in "Champions Online" - the staff made hotfixes over hotfixes so much that the expression Flavor of the Month was reduced to Flavor of the Patch, as in classes being completely revamped every few weeks, their skills being changed drastically every few weeks and all of that just due to whining in forums.
All of this is parts of the reasons why SW:TOR still amazes me and keeps me hooked. Yes, the magic of the first half year has faded - but now I enjoy the harder content more and more and I take my time about it, as does my guild. We like things close-knit, familiar, on our own pace. That does keep the magic alive on quite some weekends and I dare say that this is also our own achievement and also our own burden. We try to make things special, try to really stand out together in the way we communicate with each other and try to achieve a shared-consciousness level tactical understanding of each another.
Sorry for the very long post but it really was needed to vent this here along with my thoughts on the various reasons for what might lack - and that is "Not much" since the main thing that lacks imho is for people to get together and do outstanding stuff together. Just wait until the first musicians among us create sw:tor songs (Midas, I miss you)