Are there rules for corporate blogs?

I subscribe to a number of blogs for various reasons. Some of these reasons include:

• to keep up to date with the latest marketing news
• to challenge my thinking with various points of view
• to be entertained
• to learn

I could add several more reasons to the list but at no point would I add:

• to receive continual updates on the various capabilities of a company

Having recently subscribed to ***** blog I have read 3 postings this year telling me how good they are and what they can do for me, the client. These postings are interspersed with some great articles but they have lost me as a follower. Is it wrong to promote your services on a blog or are there no rules? Does anything go? It is quite apparent that more than 1 person in the organisation contributes to the blog. There is the easy writing style and considered opinion of the article writer and then the structured boring corporate updates.

Is this another example of a company trying to use new media but in a traditional way? Will the company accountant ever be happy paying someone to write down their thoughts with no obvious return?

Social media start up, The Population has 3 blogs on their home page each of which are the thoughts and opinions of their authors and not necessarily those of the company. They are varied, informative, entertaining and have minimal self promotion. As a result they are more believable and taken as a whole show the company favourably. Is this the way forward?

What then makes a great blog? Are there unwritten rules? Should one person be solely responsible or can several people contribute to a single blog? I guess there is a blog out there that can tell me...