Dear Johnf3f, please provide us with the filter brand and model of the filters that you're talking about that degraded the images of your photos so that we can better understand your experience. I personally use B+ W filters with nano coatings and have found no image degradement to take place. Or if there is one, I certainly can't see it. You're certainly entitled to your own opinion about using or not using filters, but when you make sweeping statements about filters degrading images, then please provide us with additional specific information so that we can at least avoid those specific filters. And I will confess to a bias for using filters. Years ago, I fell down some brick stairs outside and landed on my SLR camera. The filter was a cracked mess but the lens was undamaged.
I re-read my post and I think I could have phrased it a bit better! I stand by what I said but it does come across as a bit of a sweeping statement - not my intention.
Firstly I am not anti filers, I wave recently completed my 100mm filter kit at some considerable expense! They have their place in my kit but are only used when necessary, and (as I understand it) UV and Skylight filters are not needed on DSLRs.
The main brand that I used was Hoya as it was pretty much all that was available locally, though I did have a couple of Kood filters. If memory serves the Hoya filters were in the Pro1 and standard ranges. I still use Cpl filters (when needed) from Hoya.
Putting anything in front of your lens is bound to affect image quality, though how noticeable this is depends on the quality of filter and the sort of image being taken also whether the photographer is concerned about it or not! One thing that is unavoidable is increased susceptibility to flare - this simply unavoidable with a flat piece of glass and can significantly limit you shooting options.
I appreciate concerns about damage protection, but having seen the damage that the filter can cause to a lens dropped onto a relatively soft surface (not one of mine thank goodness) it can work both ways.
It is interesting that when Canon introduced their Mk2 SuperTele lenses they omitted the front filter element