I've given lots of presentations, and since I go to a lot of conferences I see a lot too. This means I see a lot of problems, where people are doing things that reduce the efficacy of their talks. I've not tried to come up with a comprehensive list, so the ones I'm raising here are just a few things off the top of my head. Like most smells, these aren't always wrong, but should always make you think.
Sentences on slides. I could say bullet points here, as that's the most common way to misuse slides by turning them into Slideuments. But I think the real rogue is the sentence. My approach with slides is to treat them as a VisualChannel, following this it's important that the audience is listening to the speaker for the flow of words. Reading sentences breaks that up.
Floodmarks. Lots of slides have repetitive elements on the slide master that show up on every slide, these include corporate logos, legal boilerplate (such as copyrights), occasionally even tag lines. The audience either tunes out the floodmark (which makes them useless) or is distracted by them (which is worse). I recommend you only put logos and the like on the first and last slide, that's quite enough to get them noticed, but not enough to be distracting. You only need a copyright statement on one slide - and even then I'd argue you don't really need it. Are your slides really so amazing that you'll lose a fortune if someone copies them?
Repetitive Pointer Movement. It's wise to always carry a laser pointer when giving a talk - indeed I suggest a combined remote-clicker laser device. But if you find yourself pointing to the same sequence of things every time you give the talk, then you should put that pointer sequence into the slides themselves. This may be as simple as an arrow or a highlight on the existing slides, or it may be rethinking the slides themselves to help bring out the point better.
Backtracking Slides. If you find yourself going back to a previous slide, that indicates the slide sequence isn't right. Instead you should copy the slide so that you keep going forwards through the deck. Once you've copied it, you can then think about whether it should be a true copy, or whether it should look different when used again in order to highlight how you are using it in the further sequence.
Slide Titles It seems to be mandatory that every slide has a title - but ask yourself if the slide's title is really needed. I've found I've often improved the visual channel by consciously removing slide titles.
Having said all this, the most important thing is your content. If the content isn't solid, or you don't really care about it, then even outstanding slidecraft isn't going to save you. Conversely an audience will forgive all sorts of crummy presentation techniques if you have interesting things to say.
For more good advice on presentation techniques, check out the excellent book Presentation Patterns.
1: In many places there is implied copyright, even if you don't have a statement on a document. Even so, it's wise to keep one on the document somewhere to make it easier should you need to enforce copyright. I have to say, however, that I haven't heard of anyone suing anyone over copyright infringement of a slide deck.