I hate it because everyone who i know who uses it takes stupid pictures with it. I bet 90% of the pics on instagram are of food. Who cares what you had for breakfast. Want to know what I care less about? How your breakfast looks with a yellow grainy filter applied to it. What annoys me is I think people take more photos than they should with it because they think adding a filter suddenly makes a subject interesting. Granted if it was used "properly" to "enhance" actual good subject matter, and the filter was properly selected for a purpose, not just because its different, then maybe it could be ok.
Also - they use wayyyy too many yellow filters. Makes everyone look like they have jaundice.
last but not least - hippies. A lot of people who use instagram (that I know) are in what I would call "hippie" state of mind. They usually have iphones / macs (which i hate), listen to crap music like the beatles, and enjoy recycling. All of the above I hate.
If someone told me the only pictures I could take from now on were on isntagram, I would give up photography right now.
When someone shows me a picture they took with instagram, my mind inadvertantly wanders off to where I'm thinking "I wonder how long it would take me to bleed to death if i punched through (insert nearest glass object) wrist first"