It's really a small % of game buyers that look at online reviews. Most people, and this is especially true for Wii owners, just look at what's on the shelves when they goto buy, or purchase from word of mouth or what they rented / played at their friends house.
Now ratings do matter for games that appeal mainly to the diehard crowd, like Bioshock for example. But games like Zack & Wiki don't appeal to many hardcore players and people it would appeal to aren't reading those reviews. I think longterm Z&W will succeed via word of mouth, and if they are making Z&W2 as reported, I think they'll have a successful series with each edition selling better than the last (assuming quality remains high).
Another classic problem is having an IP that just turns off the people your trying to sell to. Pyschonauts is classic example here, Dewy is another. Too kiddy for adults, too hard for kids. Z&W fall into this crowd as well. MLB Power Pros is another. It'll be interesting to see if Little Big Planet suffers from this too.
I think Drakes Fortune may suffer from this as well - It looks like a Laura Croft rip-off. And assuming I haven't read reviews, I'd skip it in favour of Laura herself. Prince of Persia is often credited with knocking Laura off her throne, but there's no mistaking PoP for LC:TR. Looks different, plays different, but close enough to steal audience. DF is also a little on the short side, and some people may be reluctant to sell out big money for a 10hr game (personally, I'd rent it).
UT3 is interesting. I'm not sure why this is failing. It's a well known, well liked IP. I know why I'm not buying it - it looks like $#!t on my laptop. UT2004 looks and plays much better. And from what I've seen/heard, UT3 is mainly just prettier, but not significantly different. That being the case, I'll just stick with UT2004, all my friends have it already anyway. It's not like you buy UT for the story or anything either.
Mostly though when good games don't sell, it's because they don't look cool or inticing to the average buyer. Sometimes word of mouth can save a game, sometimes not. Often they are too niche and the marketing department doesn't know how to reach the intended audience. Some are just slow burners (Guitar Hero started out this way). Some suffer from an overcrowded marketplace and just aren't different enough to stand out so they get left behind. We all want something that seems 'fresh'. Likely with games like CoD4 and Halo 3 that's what's happening to UT3 and Orange Box.







