1. "Your profile picture is delightful"
Why this isn't something you should do: Commenting on anyone's photo, even if it's an avatar is creepy. It's particularly creepy if you are a gent, and the person you're addressing is a lady. It's creepiest when you are a gent of certain years, and the lady in question is much less-seasoned. Be aware of the fact that what you think is funny and charming may not be perceived as such. The shorthand for this is "read the room" and an increasing number of gentlemen are discovering they don't do that very well.
2. "My novel is about Batman/SuperGirl/Wolverine"
Why this is something you can't do: Those characters were invented by, and are owned by someone else. You don't get to SELL stories/novels using them. There is such a thing as fan fiction, but it's not what we sell.
3. "You come highly recommended on the AAR website"
Why you should avoid this: The AAR site doesn't recommend anyone. They list members. Membership in AAR may be why you chose to query the agent but that's not what you wrote. False flattery is icky.
4. In lieu of the "letter" I am sending you.... (attachment) and other stuff.
In lieu of responding to your query, I'm not.
I ask for what I need to assess your work.
If you don't send it, it tells me you're going to be a pain in the hat to work with.
5. Did you get my previous email/query (#1 above)
The ONLY way to follow up on a query is just to resend it.
If you can't figure out why I didn't reply, I have a handy check list
Two of these emails and I flag your email address and it's diverted to trash.
6. You'll probably reject this but..
Publishing requires ironclad determination and tenacity.
This kind of "woe is I" sentence tells me you don't have it. I don't sign writers that
demonstrate they're going to need this much hand holding.
7. My friend says you're great
it's true I am, not telling me who said so makes this ineffective
sucking up. On my very best day (and there aren't a lot of those) I
despise sucking up. The only thing I despise more? Ineffective sucking