Buying a Wii. Such a waste of money. I know I shouldn't say that because I loved Super Mario Galaxy 2, but that's it. Way too much money spent on something that gave me a week of fun. Not doing the same with Wii U unless Nintendo steps it up, but without trophies, with already limited third-party releases and performance just on par with current gen, I'm not about to repeat my mistake easily.
Unless they do Super Mario 128 and it's the most awesome platformer ever.
There are still some good third party games on the Wii, they just do not get a lot of exposure, and are more so minor releases. I barely play my Wii too (Or my other systems now that I have a nice computer), but I know there are still some good third party games only for it, even if they are not my style. I still play Rune Factory: Frontier from time to time, and I can only play it on the Wii. There are still great games, just not the major budget third-party ones on other systems. There are still loads upon loads of shovelware to sift through to find those good ones though.
As for me, I would say playing Sonic 2006 (I did so with no knowledge of what it was like in 2006), buying Street Fighter 4, and spending way too much time and money on FTP MMO's are my biggest mistakes.
my biggest mistake this console generation? getting a 360 and ps3 late in the generation....but as far as games? well i dont regret any of the games I bought (even games no one scored well, but I personally loved, like Duke Nukem Forever, Brink, Alpha Protocol, and Resident Evil:ORC) except 1.... Karaoke revolution for the 360.... but it's my wife's (she doesnt play these kinds of games often I swear....she plays jrpgs mostly) so I can't get rid of it :P
Big difference between something being great and it just not being your genre. For instance I dislike FO3, FO:NV, Oblivion and Skyrim because I dislike that kind of game. It doesn't make them bad games, they may be amazing if you like that type of game. I find it incredibly boring walking around a giant map virtually directionless just trying to find things to do. Some people can't quite seem to make the distinction between something they legitimately think is bad and them just playing a game in a genre they dislike.
Only time I started making uninformed purchases was when I switched to mostly PC gaming and sales are plentiful. I've bought plenty of games in the last 2 years I hated but meh I spent 2.50 on it so what do I care. Back when I was forced to pay stupid prices for console games I made sure I knew what I was getting into before hand. Now I just buy whatever sounds interesting if it's cheap enough.