The first thing I tend to notice is the amount of damaging or "triggering" posts, anything from eating disorders to mental health, depression, sexual fantasies and more. The anonymity of the site allows a lot of people to "come out of their shells" and start to talk about what's on their mind.
Unfortunately, when you take away the shame of public and open speech, you tend to find that the worst parts of humanity start to see through - there are entire blogs devoted to losing weight, anorexia and pictures of women with impossible physiques, creating a circle jerk culture of unhappiness and lustful ideology.
Of course, in regards to user experience, the content is user created, so if the prevalence of these kinds of posts says anything it is that this is what the people who use the site want to see, judging by the tens or hundreds of thousand of re-blogs they tend to receive.
However, if you ignore the seediness of the anonymous world for a moment, you can also see the good in the site, there is a lot of art which would otherwise be hidden from the world, beautiful poetry and pictures and there seems to be an inherent nature in the people to stick with each other, giving one another support and advice whenever they need it.
This can be seen especially in terms of suicide and depression, where there are people posting about their self harm troubles and other people sending them "asks" to support them and tell them it will all be okay, it becomes somewhat of a massive online support group. These people don't feel like they can talk to people in the real world, their counselors and family don't understand so they come online for their fix of advice and help, to actually feel wanted and loved and cared for.
This is something which websites like Facebook and Twitter tend to miss, for better or worse, our profiles there are normally tied to our identities, we can't talk about these private things because then people in our real lives will know our most inner most fears and troubles, the sick people behind our normal exterior, so they use Tumblr to express this part of them.