Things I like about newegg:
1. One stop shopping. 99% of the pc stuff I buy, newegg has. I like one stop shopping, any of the hassle I can remove from the shopping process, I try to do.
2. Reliable processing. I've had 15+ orders go through, not a single hitch, not once. After dealing with other vendors, you get to like that peace of mind. I can order something and *KNOW* that my order got processed and will ship without anymore communication from my end. Not only that, I'll get an email when it is processed, when it is shipped, and when they have a tracking number.
3. Good prices. Are they the cheapest? Usually not. Is it worth my time to nickle and dime and each part, ordering from multiple vendors, hell no. I bill out more in 1 hour than I can save by trying to find that absolutely best deal for each item. With a few notable exceptions, newegg generally has pretty good prices.
4. Good reputation for RMA and service after the sale. I've not had to utilize these features, but the customer reviews from those who have speak for themselves.
5. No bullshit. It is either in stock, or it isn't, at the time of my order. Very little pisses me off more than for something that was 'in stock' at the time of my order suddenly becoming 'backordered'. I decided to pay cash for a camera body at a local shop the other day, whose website said "in stock" before I left my apartment. Amazingly, they went from plenty of stock to no stock in the 10 minutes it took me to ride the subway to their shop.
Basically, it comes down to the entire shopping experience, and newegg is hard to beat for the total package, imho. I used to use mwave quite a bit too, but their pricing has been terrible the last few times I looked at stuff.
Ha, one other bit of info, newegg has advertising on the sides of nyc buses now.