YOU COULD BE BUYING THIS!
Large, jarring adds or elements in the UI that are constantly pointing at something like a heckler on the street, tend to pull me away from the fact that I am supposedly playing a game and instead remind me that this interface and experience exists entirely to get money out of me. Now certainly on a high level that IS the main intent of any game, subscription, buy-to-play, and free-to-play, but like any time you are trying to get something from someone, the best way to do it is when they are not even aware that you are. People have paid thousands of dollars to Blizzard to have some computers switch a few values in a database, at $20+ per transaction, without a second thought, as though it was some kind of convenience.
Instead, some games, like SWTOR, wave their F2P aspects in your face, and it becomes clear early on that you are just there as a customer....not to have fun. Imagine going to Disneyland and having the vendors follow you around offering things constantly. Much less fun right? Yes, the idea that I am anything but a customer is merely an illusion, but so is much of life. The better the developers keep up that illusion, the more fun I will have, and, ironically, the more likely I am to spend money...especially since getting that first dollar out of me is the hardest part.
So if you are making a Free-to-play game, certainly it's alright to have things for me to buy...that's your model after all, but I don't want to feel like I am at a bazaar or passing an endless stream of sidewalk activists as I play.
Cutting off your nose to spite your face
I hate to be beating up on SWTOR but it does seem to be the offender I can cite for my next example. When transitioning from a subscription to an F2P model SWTOR moved many services and functions (even some like "Hide Helmet") to a purchase model. They did provide previous subscribers with the currency needed to buy all that, but (at least for me) the currency could not bring me back to the level I was previously at. I don't expect everything for free...but SWTOR's UI had been hemmed down by the removal of action-bars, and a constant reminder in the corner of the screen reminding me that I could pay to remove these elements.
In this case, I think they went too far; it goes back to my first point of not wanting players to always feel like they are being sold something. In the case of SWTOR, for returning players it was very clear that they had lost a great deal and had to subscribe to get it back. So I am getting essentially a nerfed experience than I previously had, and while I used to pay for it, transitioning to F2P somewhat implies I won't need to "as much". We can quibble over whether it's justified, but to me it became a turn-off from returning.
I began to feel like I was playing a demo with strict restrictions placed on it, treated more like a "visitor" than a player or customer. I understand that the point of F2P is to make money, but this was a case of putting far too much behind the pay-wall. The list of things you have to pay for access to in SWTOR is dizzying, and somewhat baffling when the biggest players in the F2P market (World of Tanks, League of Legends, etc) hide relatively little gameplay behind the cash structure. They lost me as a customer (and I am allowed to be capricious and fickle, this is my leisure time we're talking about here), mainly due to what appears to be somewhat greedy behavior, when they might have gained me back if I felt a little less shortchanged (I would probably have been willing to pay out for Cathar if I had not been also feeling like I had to pay just to hide my ugly hat or match my outfit's colors).