Some of my own observations:
For starters, both Steven's blog and Mark's blog (and all of our recruiting blogs) have primary or secondary goals of recruiting readers to work for their respective companies. So, don't take everything that you read at face value. Ask a few people who work for Microsoft, and ask a few people who work for startups. As Dare pointed out, the truth probably lies somewhere in the middle of what these two say. Also keep in mind that in order to be applicable to a large number of people, this dialog needs to remain about "Working for Microsoft vs. working for a startup" -- not "Working for Microsoft vs. working for Plaxo". Your mileage may vary from startup to startup.
I have a unique perspective because I've worked at 2 startups (stamps.com and Xtime) out of college (one internship, one full-time), and I've worked at Microsoft out of college (after returning to finish college). In my experience, and again, while your mileage may vary both across startups and across groups and positions at Microsoft, I was able to make an impact at all three places.
That said, at Microsoft, I've felt expected to make a much larger impact than anywhere else I've ever worked. When I first started, I would raise questions like "Which architectures should our product support?" I was quickly told, in every case, something along the lines of "We pay you to tell us that. Go figure it out." I work on a brand new product, and have impact (read: decision-making power) over much of what we do or don't do and how we do or don't do it across at least three large feature areas for which I am responsible. I also participate in, and influence, the development of features and products I'm really not responsible for, simply by weighing in and providing feedback when I'm so inclined. In my experience, having this type of impact at most startups straight out of college is unheard of for the reasons Steven cites.
On grunt work:
At a startup, you will do grunt work. But you'll probably enjoy doing grunt work due to the high pressure, "make or break" situation of which you'll find yourself a part. It's like Mark says - everyone does grunt work when it's necessary to get the job done (in a healthy startup, at least).
At Microsoft, there is grunt work too. There's less grunt work, because the role of a Microsoft employee is usually more specialized than the role of a startup employee. I know about 20-25 college hires, and very few are spending a significant amount of their time "taking care of build scripts, fixing code in bugs they didn't write, and setting up test machines" (and certainly no more than they would at a startup). For example, in my division, we have test labs, with test lab systems engineers and network engineers that specialize in being the best at designing and building out test environments. As a Program Manager, I'm paid to plan, design, and drive implementation of new software features -- not set up test machines (though I did poach and set up a pile of "test" servers in my office purely for fun - *whistles innocently*).
At both startups I worked for, I spent a significant amount of time setting up test machines (in addition to programming, and doing a very wide variety of other things). In my opinion, one situation really isn't better than another - they're just different. You need to decide which you would prefer. And as for grunt work - there's grunt work everywhere. The best workers are the ones who identify it and step up to do it without being asked - at a startup or at Microsoft.
This part of Mark's post really gets me:
if you want to know what working at Microsoft is really like, check out Mini-MSFT’s blog and the comments people leave there. I guarantee you that’s the real deal.
Let me tell you a little story. Back at the University of Michigan, students would protest a wide variety of things day in and day out. We'd debate the University's affirmative action policy. We'd debate selling Coke due to the company's negative environmental impact on water sources in distant parts of the world. We'd actively protest budgeting decisions. Every student government candidate would make extending the campus library and gym hours a central issue to his/her campaign.
Some of these issues were serious. Others were silly. Either way, they made big headlines - in our school newspaper, our local city newspapers, and sometimes even in larger national publications. If, without any context, a UofM recruiting candidate were to be thrown into the middle of one of these chaotic debates with some students literally crying over their "oppression", he/she might think the student body was oppressed, and the University simply a messed up place.
In fact, the opposite is true. The University of Michigan is a great place because of the conflict, the debate, and its tolerance thereof. I'd draw similar parallels to Microsoft. At no other company will you find such an open environment where people openly debate everything from the company's policy towards gay rights to the company's policy towards free clean towels in the locker rooms (nevermind the fact that Microsoft's buildings even HAVE locker rooms!).
Unfortunately, a side-effect of having 60,000 bright, rambunctious protester employees, is that dirty laundry will be aired publicly - no matter how insignificant or unrepresentative of the general situation. When Microsoft recruits come across the Mini-Microsoft blog, they get a skewed picture of reality -- despite the often insightful writing and comments. I'm still not sure whether or not I think the benefits of Mini-Microsoft outweigh the negative impact it has on recruiting, branding, etc. I highly support efforts by Chris Jones and others to create internal "safe" blogs that take on the hard issues and promote debate, and wonder if these provide a sufficient middle ground.
I think that Steven Sinofsky is going a bit overboard by removing comments that link to Mini-Microsoft. Mini-Microsoft is credible. Sadly, I think the reason some people detest the blog so much is largely because it is in fact credible. Anonymity doesn't affect the credibility of the actual information. If the information is wrong, take on the information, not the author (especially in this case, where Mini-Microsoft has actually proved himself to be rather intelligent and engaging).
And if you do believe that anonymous information cannot be credible, it still leaves me wondering... when did lack of credibility become a reason to start censoring comments on a blog? Blog comments that aren't credible still have the potential to promote insightful discourse and debate.
Anyhow, just my 2 cents. I was going to leave it as a comment on Mark's blog, but it got a bit long-winded. Sorry 'bout that.