Gaming sins... hmm. Not sure I really have very many.
Well, for starters, I hate Resident Evil 4 and Resident Evil 5 as RE titles. As games I have nothing particularly wrong with them, they're actually pretty fun to play. I just hate them as part of a series that started out so completely different, both in terms of gameplay and in terms of atmosphere. The Fallout series took a pretty big change to gameplay, but at least the atmosphere and style haven't changed. The new Fallout titles at least feel like they're part of the same series. Resident Evil on the other hand has turned from campy survival/horror to incredibly over-the-top cheesy action complete with one-liners that would make Arnie facepalm. It makes me sad.
And hey... speaking of Fallout... I don't like Fallout 3 all that much. I consider it the second-worst game in the series, only better than Fallout: Brotherhood of Steel for X-Box/PS2. Granted, I consider Fallout 3 to be many, many leaps and bounds better than that atrocity... but still. Yes, that means that I even liked Fallout: Tactics: Brotherhood of Steel better. I just really couldn't get very immersed in Fallout 3. I felt no real attachment to any of the characters, I felt no urgency whatsoever in the main questline, I found the continuity errors raised by the Capital Wasteland branch of the Brotherhood of Steel to be significantly worse than the ones brought up by the mid-western branch in Fallout: Tactics, I found the game to be lacking quite a bit of the dark humor and pop-culture Americana-based humor of the rest of the series... I dunno. It wasn't a bad game or anything, I just found it to be a pretty weak Fallout game. Liberty Prime was hilarious, though.
And last but not least... Call of Duty: Modern Warfare 2. Not only do I love it and still play it, but I play it in ways that the vast majority of players would seem to refer to as sinning. Do I camp? Absolutely. Especially on Domination maps once my team has taken a second point. It's more important to secure and hold the points that we have than it is to take the third point. Especially since taking the third point is just going to scatter enemy respawn locations and make it that much harder to hold what we have. Do I use the M203 grenade launcher, dual-wield extended magazine Glock-18's, suppressed UMP-45, the ACR, extended magazine AA-12, and thermal sighted sniper rifles/LMGs? You bet. Why? Because they're effective at the roles they fill. I'm not a particularly good player to begin with, I'm fully willing to admit that, so why intentionally handicap myself further - for my enemy's sake? With the goal of each match being to win, I'd want my teammates to use whatever weapons work best against the opposing team, regardless of what those weapons are. If the opposing players don't like it, what do I care? My job is to shoot them in the face, and I'm going to use whatever gun happens to be the most useful for face-shooting.