Using only one opening?

I always open with the Queen's pawn / ...this is all I use / ... I rarely ever lose when playing as white/ ...and my opponents have jokingly critizied me on being a one trick pony when it comes to my openings.

Is there anything wrong with always starting with the same move? " 

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YES, there IS Something-Wrong with it, imo, tho being only restricted to 1.d4 still leaves you a lot of territory to vary your play & no doubt you could play with some level of success, for many years without ever venturing 1.e4 or any other first move than d4. Perhaps great satisfaction as well. But if it were me, I would find myself in the circumstance, that I would Always "Wonder". "What IF" I HAD gone ahead with learning alternatives & perhaps playing them. What IF?

       Before more on what I feel might be the Wrong of adopting a narrowed approach, I'd like to proceed to consider what is RIGHT with it. As you suggest, it allows great familiarity (eventual boredom, in boardom?!) & comfort with the lines of play. Often a very good approach to learning to play high-level Chess... To first become expert in a certain opening & position type(s) coming from it. Then as you approach mastery of it, beginning addition of yet another system/opening to your repetoire & understanding. Your play should be quite likely much stronger in your Choice Opening(s), when someone does play into it/one.

 [eg. When playing serious Postal Chess, my rating only got to the lower half of the 2200's by the time I retired from it. But if anyone played into my King's Indian Defense it had a "Performance Rating" about 2285 for the games it was used for. Or facing my best, the "French Defense" they must meet an opening where my Performance Rating {a means of determining skill level displayed in a particular grouping, such as a certain event(s), or for all games using a certain opening, only. Etc.} for use of it was over 2450 at the time. Needless to say, this was beneficial to results & sometimes may have had a bit of surprise factor to an opponent. 

    Not only study, but as you suggest, Playing that opening only ... or very frequently, benefits most. Then you eventually meet all manner of attempts & ways of trying to counter it, thus strengthening breadth of understanding, as well as the length of your related knowledge coming from postmortem of each game played. Errors determined in such review are replaced by superior moves aor Plan Idea(s). Next time that line is met, your correct play goes further than before. 

    One Chessfriend & OTB tournament player I know was at Expert rating level (a.k.a. "Candidate Master") when he decided to announce to all that he would be trying to reach Chess "Master" by limiting his repeatoire until attaining it. He would play only his best openings then. The Najdorf Sicilian Defense, his King's Indian, & only 1.e4 as First Move. If opp's wished to study-up for playing him, they need only learn/improve in those. Consequently, some players did BOOK-UP on their theory of those lines, to better play him. And that DID happen too. Exactly as he wanted~! Thus He was both "Forced to" & concurrently Given the "OPPORTUNITY TO" get better, having gained opponents could then play against him much deeper into the game without erring, than they could before. Thus he got to play against not only the opening sequence, but then also the most common and less common middle games resulting within his repetoire. And the Endgames too. He DID reach his Master rating in a short time  :) 

    It worked especially well for him, already having years of past tournament experience. Therein he had played many varied openings & types of positions; thus had the beginnings of a rather broad & comprehensive understanding of Chess. And simply needed then to improve in the openings/positions he already favored & probably played his best in.

    And so, if you do not now have THAT basis as he did, lacking it will be the "WRONG" that IMO may occur if you stick only to your single "...pony" opening. You will miss out on the learning, familiarity, & confidence needed for you to play position types less often seen from your chosen opening ... but still may occur during competition & may challenge you re: how to proceed with it; or at least costs you great Time loss from your clock as you try to figure it out 'at-the-board. Matters that could have been learned previously thru study & use when playing Club/Skittles games or other less important ones. Having become pre-educated in them tho, would not only give you a much broader base of position types & IDEAS to use for comparing to the situation in a competition game you are playing, but having done that would also give you a greater feeling of confidence in your comprehension & play. 

     Also the IDEAS learned might not only be of use to survival if your opponent gets you into one ... They/It may well be essential to your ultimate standing as a player since KNOWING them you may find you would actually WANT to play into such position & use the idea(s) within it, during some game you are playing now, because of that idea being one which will WIN or offer Chances (that you would not even have known of nor considered, had you never played in any such before; being uncommon to your usual d4 repetoire).   

     In addition, when you do go into new areas of exploration for opening or variation choices, you might actually find that you play THAT type game even better than your present chosen few. Whether that is just a natural superior ability you have for the new one, or that it happens after you begin to study into it aor use a bit. 

     It can be useful as well, to have secondary opening choices to fall back upon if you are in a tournament for instance, and some early opponent would happen to destroy a line you were currently using, in the thought it was playable ...but now see it needs some intensive analytical review before it could even begin to be resurrectable for you. It hurts twice as much to lose a 2nd game to the same variation later in an event, just because someone saw & copied your prior opp's play...or just figures out the same Improved line that he did. Also a secondary is useful for avoiding some particular opponent line that you just prefer not to play thru, or perhaps find to be too drawish, etc. & to keep other thinking about what you Might play. Suppose you do prefer positional play to highly tactical games & an opponent considers maybe you do not have experience in the Tactical areas. If you don't, it will put you in a tight spot when he decides to do everything he can to steer the game into opening & middlegame tactics. And if he attains that goal, putting you into less familiar territory [maybe being less comfortable territory as well!?] then you will find yourself having to make many more decisions upon which you have little experience to base you choice(s) upon, thus creating more opportunities for you to choose wrongly [err or even blunder]. And that is just what your fellow across the table was playing for~! 

    What you Don't Know CAN Hurt You . . . 

Regards All, Craig A.C.