Tough question. I prefer upbeat, happy blogs because I myself am a happy, upbeat person. I don't really have any reason to bitch and moan, so I don't. The most I'll vent about is a lack of a job or something that's pissed me off. But those are normally major subjects (like a co-worker treating me like shit, etc.)
On the other hand, if I find a blog I really like and it is someone's personal blog, I will definitely still read their blog if there is a depressing or negative blog post. People go through depression and hard times and I do not think they should censor themselves on their own blog. So I'll read them regardless.
I try to stay away from negative blogs and making negative posts myself. Unless I've been reading the person for a while and they are going through something I might learn from I usually unsubscribe. I'd prefer to surround myself with positivity and it's always nice to read an upbeat post when I'm having a bad day.
Generally I prefer reading upbeat or positive blogs as opposed to reading the rants of a disgruntled 20 something blogger, but I don't hold it against anyone who does that. A blog is a great place to vent and I do that on occasion.
I go either way. I really like it when someone is upfront and honest about their life. Of course nobody likes reading constant sad or negative blogs, but if you mix it up it seems more real. I just blog about whatever is on my mind. But the happy posts usually get a better response. www.lifehandinhand.blogspot.com
I write how I feel. When I'm very upset about something I express that, but if I'm feeling really positive that's what comes out in my posts. If I'm just ranting about something I try to inject it with a few jokes or something to lighten the mood and let my readers know I'm not seriously wrecked about whatever it is.
In terms of blogs that I read- If a blog was always happy-slappy I'd probably think that the blogger was hiding something or being insincere, and if every post was about how depressed the writer is I'd feel bad for them and wonder if they had any sense of humor about anything.
It's all about finding the balance and expressing a full range of human emotions.
I don't think that it's really one or the other really...I think blogs, if you're being honest in it, follows life which encompasses all the ups and downs that go along with it. I've had some sad, depressing blog entries in my just over 2 months of blogging, but I've had some really happy, funny blogs as well. I completely agree with Heather in that we are human and we do have emotions...no one wants to read a perpetually happy blog...it ends up seeming almost fake at times if the person is always upbeat without a hint of fear or worry or sadness or something. Personally, it saddens me when I read those blogs that are "happy happy" all the time...it makes me feel like the author doesn't really know who they are as a person. I agree with Angela - I try to have a more happy, upbeat tone to my blog, but sometimes if crap hits me in the face, then that's what goes on the blog and I think both as an author and a reader, it's a lot more real to read about that.
I think a lot of bloggers are going through some difficult things, and use their blog to vent about that. Sometimes it can be hard to do it with people in your real life and you use the blog as an outlet. I do that myself, a lot. I find it quite interesting to read how different people deal with difficult situation and I learn a lot from seeing how people respond to them.