I agree with not using tags but not for the reasons you cited. There's no worry that it will look like spam. Tags are built into every blog in the world and everyone understands what will happen if you use them. To penalize their use would be crazy.
The best reasons not to use tags as they don't serve any real purpose. The operate exactly like categories. So you end up with a bunch of categories and also a bunch of tag categories. If you really do have related content then provide links to those articles, install a related posts plugin, or build a better category tree. Liberal use of tags just make for a messy, diluted, unfocused site. Now that WILL affect your rankings.
Comments can definitely help with a pages rank if the comments are on topic. Users will add all sorts of additional content and keywords that turn out to be quite helpful...especially from expert commenters. They sometimes write comments that are better than the original article.
I always give my commenters a dofollow link for their name because I know that some of the best commenters are also SEO savvy and there's no reason I shouldn't reward them with a little link juice. Spammers get trashed immediately, and if someone's comment/keyword name is borderline I generally just strip out their hyperlink.
Just focus on a making a clean site with a good architecture and write great content. Does anyone actually make a blog because they have a passion for something and just want to write about it anymore? Seems like every blog question is an SEO question and the blog in question is some marketing gimmick. I'm continually amazed that people will create blogs just to sell adword or affiliate links. That's not something I want to be remembering on my death bed. I'm hoping to leave a great body of work and make some sort of contribution to society.
"Grandpa, what are you most proud of?"
"Oh I remember one time *cough* I spend three months paying writers in India to generate generic keyword rich puff pieces while simultaneously paying a Russian team to generate thousands of crappy keyword rich hyperlinks. I tell you Bobby, *cough* that site made $500 per month, for eight months, until the Panda update wiped it out. Yeah, that was something."
"Uh, I really don't understand any of that. Anything else grandpa?"
"Oh, and the day *cough* I married your grandma."