Excessive Ads & Ad Tracking Scripts - Not only are ads intrusive and break up the content of your site, as well as annoy your visitors, they can also slow down your site considerably and be a security risk to your audience. In addition, many of the conversion tracking scripts and analytics scripts related to them will also slow down your site. Platforms such as DoubleClick will generally piggy back all sorts of tracking codes from other ad networks into your page as well. So you may think you just have the DoubleClick tracking code on your site, but it might be loading up like 20-30 other tracking cookies when a user visits a page on your site, and slowing things down. Google has also had as few screw ups in the past couple years where Malware was sneaking through their ad network and infecting computers where their ads were being displayed.
Site Design Isn't Mobile Friendly - This is a big mistake, not only does Google frown upon this, but your bounce rate will only get worse as more and more people are browsing the web from mobile devices. In some cases, sites are seeing more than 40% of their traffic coming from Mobile, it can't be put off anymore.
Franken-sites - This is something I am seeing a lot of too, where complete newbies are creating Wordpress sites and adding a ton of plugins for every little minute thing. While it's nice how extensible and adaptable Wordpress is, it's also easy for amateurs to get carried away with plugins and widgets to the point where the whole site is a slow loading, sluggish, piece-mealed together mess. The margins are a mess, content blocks are accidentally overlapping, inconsistent fonts, five or six instances of JQuery being loaded up accidentally, mulitple instances of Google Analytics, etc... It reminds me of the days of MySpace pages, where people were putting up annoying music on their pages, hundreds of tacky animated GIFs, enormous wallpapers that took forever to load, awful fonts, etc...
Not designing for users who are blocking ads - Lets face it, there are a substantial amount of people out there using plugins such as AdBlock and AdBlock Plus. So much so that large companies such as Google are actually paying off the creators of AdBlock Plus to not block their ads. A visit is better than no visits, regardless of if they are seeing ads or not. Don't throw the baby out with the bath water by blocking or impeding visitors who are using AdBlock. Your site's content blocks should collapse gracefully when the ad blocks are empty.