We are lucky really as tarot people, having this great forum here, because if we get decks that we regret buying it's easy to find people who will trade with us or buy the decks we don't want. Also tarot cards can be worth holding onto even if you don't like them, as they may become valuable if they go out of print.
I seem to go through a period of regret with nearly every deck I buy; I like it so I buy it; It comes and I use it a lot in the first week; Then something puts me off it and I regret buying it and it goes in the cupboard; Later on I come back to it and see the value in it, sometimes after reading a post about it from someone here.
I agree with there being nearly always a card that you don't like in a deck. The latest one I found was The Prince of Sceptres in the Vampyre tarot. It's something about the way his body his arched and his muscles are drawn in his abdomen- it just looks wrong to me somehow and it irritates me every time I get it in a spread! I know that sounds silly, like I should be objecting say to the 3 of Pierced Swan in Shadowscapes or something (I actually don't, but I can see why people might), at least something with a real reason.