Unlike conventional desktop / notebook-based computing, Mobile browsers has greater limitations including screen / resolution, processor / hardware capabilities, user & environmental factors.
To overcome all these limitations to maximum possible extent, the application needs to follow certain guidelines while designing such applications. To start with, I am contributing some stuff what I have learned so far. I would like you all to contribute on 'important aspects' while start designing a mobile web / application.
Important factors while designing for mobile web / applications:(I have referred the mobile web app which I have developed for my company, bcwebwise.)1. Address Users' Requirements As Quickly As Possible:Mobile web / application users are the ones who are using the web for their immediate needs, unlike desktop-users. For e.g. locating the nearest cafe or finding the closest Theater or for a short-spanned entertainment. For Such users, "quickness" makes a lot of difference, time being a critical factor. In such cases, understanding the users' immediate requirements & assisting them to fulfill them quickly enhance the experience of your application.
2. Navigation:Screen "real estate" is very valuable & precious when it comes to mobile devices & so, needs to be used wisely. Mobile devices have varies screen sizes. For such limited screen sizes, what gets delivered upfront, "above the fold", does make a difference.
Ideally, DO NOT repeat the main navigation on every page, as it pushes the content down, generally "below the fold" making it inaccessible for the user. Also, make sure you place it on the homepage as that being an entry point to the site.
3. User Inputs:Entering text on a mobile phone can be more painful than on a conventional desktop-based input mechanisms. Make sure your design allows the users to input the text in an easier way. For example, allow them to choose by selecting an item instead of manually entering the text (as it is even more error-prone). For example, a Telephone no. displayed can be used to call the particular no right from there itself, instead of copying the no or manually entering it on the dial pad.

(In this example, contact numbers displayed can be used to call the respective ones directly from the link.)4. Only essential stuff:Provide only essential & relevant information on every page. User's generally pay for all the content they see on their devices & so, for them, any inessential data shown is a pain, because THEY have to pay for the content they DON'T want & gets frustrated. Content designed for mobile web also needs to be crisp & to the point, as the users will not be interested in reading typical junk on their devices which they are not interested in & have to pay for it.
5. Basic controlsMake sure you place basic controls to navigate on the site, such as "back to home, back to search results" & relevant ones. Putting "Back" on every page is a good practice. Use contextual menus if required & only if they are assisting the user (preferably limited to 2-3 short links).

There is lot more when it comes to Usability for Mobile Applications, but I would prefer to put it in the next post & would cover as much I know. Please provide your feedback on this post. Your suggestions are welcome!