One bad design decision can ruin an otherwise good experience.
The other day I tried out Threadsy, an email and social-stream aggregator. I was immediately impressed. It's really nice to not have to go to 4 different places to keep up with all your emails / notifications / direct messages / etc.
After I had been using it for a little bit, I got this pop-up:

I actually was really liking it, and was already thinking about Tweeting about it, so I thought "oh sweet, they just made that part easier for me as well". I clicked the button. It spun for a second, then gave me a thanks message. I thought to myself, "Thanks? Wait, what? That's not what I expected!? When do I get to share how stoked I am about this app?"
Hoping I was wrong about what just happened, I opened up Twitter and sure enough:

Which I immediately felt compelled to follow up with:

Now, it's pretty easy to argue that it's my fault for not guessing that would happen, but that still leaves a horrible taste in my mouth. I wanted to tell people that I thought there was a cool app they should check out. I definitely didn't want to tell them about some spammy-sounding t-shirt contest, nor, more importantly, did I have any indication that I was about to. The first I had heard about a contest was by reading the tweet I just posted. It's annoying for any app to do that, but it's extra-ridiculous when it's an app that wants to manage my social streams. It's a pretty negative blow to the trust I have in them.
That's a bummer, because aside from that, it looks really cool. One small design decision pretty much ruined the good experience I was having with that app.
To be totally fair to them, they did respond to my tweet really quickly, saying they were sorry and that they were removing the preview-less share pop-up. And honestly, this isn't the most egregious breach-of-trust I can think of. But it really does illustrate the point that first-user-experiences can be tricky and designers should take just as much care with them as they do with the rest of the app.