I asked this question on Quora last week here is the best reply.
1) Advertising - Both on the App Store and with all the various news sites, blogs and app-sites. If you get App and game of the week, I definitely will at least bother to see what the fuss is about.
2) Ratings not reviews - The stars specifically, not so much specific reviews unless it's a expensive app. Unfortunately, even though we know user reviews are usually unfair and flawed, they provide the most accessible form of filtering the good from the bad.
3) Promotions - When you go from $5.99 to $0.99, it becomes a buy-first, think later purchase. This is particularly true for games. I noticed I tend to buy games that I know I will regret later or stop playing after a short while when they are at that magical price point.
4) Word Of Mouth - Most people, including me, still get app recommendations from other users. Hence, it creates a situation where the popular apps get even more popular. It does not have to be the best, but if I see it's a acceptable solution, it is good enough. E.g. I found out about Goodreader and Whatsapp from other users, and I've found that I myself am heavily recommending them.
Thanks to Khoo Wee Sheng, you can continue the discussion here