It has been said in various degrees, but I can't stand flashing ads - I hate to do it, but I had to use my adblocker on NS because I get after images really really easily (hyper-sensitive to bright lights of any sort - it's a thing) and it really messes me up haha. Some icons from the team updates mess me up too, but what can ya do.
I don't mind the background ads, they are actually nice because I can choose to either look at them or not, and they are usually aesthetically pleasing.
Ads that expand are extremely annoying - if you want to keep this an option for advertisers, they should only expand when you click on an expand button (if this is already in place, sorry, I must have spastic fingers haha).
Video ads aren't bad as long as there is no sound and they stay within their confines (i.e. don't expand unless I want it to).
Pre-roll ads are annoying, but if they were shorter it wouldn't be terrible. 45s is really long. If you don't have my attention in the first 5 seconds, then the next 40 are wasted; shorter, more salient ads (this does NOT mean FLASHING) are going to get my attention. Plus, if I see that an add is 45s I will click off to another tab for said amount of time and then come back when the video is ready. If it's a short ad, I'll stay for the whole thing.
The regular sized banner ads are generally unobtrusive.
I don't mean to rip on advertising so I hope that's not how is came across. I am actually stoked that so many people want to advertise on NS because it just shows how far we have come in the past 10 years - from needing donations by members to having large companies fighting for "prime time" NS. Not to mention that the improvements to NS as a result of this is so worth it in the long run, no matter how much I may yearn for 'the good old days'. When I do that just ignore me. I'm being sentimental.