Prevent ads from being louder than the song that came before it. I know the point of the ads is to annoy us into paying up, but it can be very painful to turn on some relaxing tunes after school/work, turn up the volume a bit to hear the quieter details, and then have a surprise advertisement blaring some awful hiphop noise or a screaming spokesman blast 5 times louder than the song. Just normalize the volume of ads so that they aren't louder than the song they follow. Ads should be the same volume as the music.
This can be implemented by having an internal variable to measure the average volume within the song, and then set the ad's volume to that **bleep**. This way, classical concertos are recognized at the lower volume they are, and the next ad won't be 5 times louder than the loudest part of the song. Or just have a "advertisement volume equalizer" checkbox somewhere in the account settings. Or let us turn up the volume for just one particular quiet song within a playlist, so that everything else plays at normal volume.
To all fellow Free users, please take my survey on what you do when an ad is BLASTED at you- I'm sending the responses to the companies being promoted by obnoxious ads, to let them know that loud ads reflect badly on them when we rip off our headphones. Also, contact the companies whose ads are BLASTED at you, and ask them to tell Spotify not to blast their ads. Shari's Berries apologized to us, and a McDonalds rep said they only design ads. Keep it up! Contact the marketing department of these companies and politely ask them to demand a normal volume for their ad.

Update Sept 27, 2017: Thank you so much for taking my survey about how you react to loud ads - it has 1,571 responses! Here are the results - please show these to companies whose ads are blasted at you!
How Spotify users react to loud ads
Answers to Other (Please specify)
10/14/2014 1:11 PM: I tear my headphones off, then lower volume on spotify, then press play (cause lowering volume to bareable pauses the ad), wait for it to finish then pause the song after the ad so I can turn the volume up again
10/1/2014 4:29 PM: I take my ear buds out, then stop listening to the service. I **bleep** my hearing.
9/26/2014 12:12 PM: Contact the company and point out that I am going to avoid their products until their ads are played at a reasonable level.
9/20/2014 12:06 PM: click on the ad thumbnail, because it makes the ad stop playing
9/18/2014 11:25 AM: go to youtube or my ipod and listen to music from my playlists
6/9/2014 5:51 AM: have a hotkey and i mute it completely, then there is a timer that umutes it after 1 minute (only for loud ads)
4/14/2014 8:47 AM: turn off, switch to itunes
3/18/2014 1:27 PM: Remove headphones to avoid inner ear damage AND lower the volume on my device + lower the volume on Spotify to avoid annoying my coworkers, as the ads can be heard across the office because they're so LOUD
Updated on 2018-02-07
Hi everyone, we appreciate your feedback here in the Spotify Idea Exchange around our ads for Free users. Please know that we have applied audio normalization to all of our ads and that all new audio follows the same volume guidelines that we have for Spotify.
If however you still find a specific ad is 'too loud' or has disruptive sound effects, like car noises, please post the specific details below as per the instructions in this thread - we'll make sure to pass it on to the right team.
Thanks again for all your feedback!