A delight to use
Some apps are just a delight to use. They may not be the quickest or have the most functions but they create a pleasing enviornment which makes you want to use them more. This can be especially true when placed in contrast with alternatives within the same catagory but is also true of brand new types of apps. If they are pleasing to use then you may well choose them over something which can (on paper) produce higher quality products.
The appearance of an app can play a large role here but so can how you navigate and interact with an app. It isn't easy to quantify and something which is “delightful” to you may be a novelty to your friend. As David sparks says, delight can triumph efficiency.
Opens whole new options
Some apps bring whole new options and functions to the table. Image the revolution when the first word processor which could add images came along! Now you could have pictures in with your text! Or what about the first online second memory bucket like Evernote. If an app can do something amazing which no other app can, it has a lot of sticky potential.
Does something basic, better
Likewise, some tools will let you do a basic function better. The RSS reader which let's you scan through quickly, the internet search engine which returns more relevant results, or the camera app which let's you take a photo quicker than any other. All these are tools which do something basic better than the rest. You might not think that the status quo is bad but after you've seen the improved tool, you can't go back.
Arrives at the right time
Time ing is hugely important. If a tool arrives when your really under pressure then you'll probably ignore it unless it solves your immediate problem. By the time you are ready to take a proper look at it, the buzz may have died down and you have forgotten all about the tool or the reason to check it out. Likewise, an even better tool could come out straight after and steal it's thunder. There are a lot of personal factors that can come into play here but timing matters.
a forced replacement
Kind of related to the above point but sometimes tools go out of service, just this morning I got a message that spring pad is closing it's doors. I wasn't an active user of spring pad but if I had been, then I would be actively searching for a new sticky tool to replace it. On the other hand, sometimes tools get “upgraded” or subsumed within other tools. These force you to switch to a new tool which may, or may not, have the same great functionality of the original. Either way something you have to replace or is replaced for you is more likely to stick.
However, just as there are factors that can make something stick, there are others which almost guarentee an application will be removed.
Don't repeat yourself
Apps which do almost exactly the same thing as another will almost certainly not stick. Who needs 5 text editors? One will probably be the main tool for 90%+ of all tasks and the other 5 will pick up the remaining scraps. Hardly likely to stick.
If an app adds complexity to a task then it's a prime target to not stick. Why use something complex when there is a simple alternative? However, adding some exciting new functionality almost inevitably adds an element of complexity. There is definitely an awkward balance here.
Removes Vital Functionality
Sometimes there is a single function within an app or tool which makes it worth putting up with all the negatives. If a new tool comes along without that functionality, it won't make the list no matter how delightful and exciting it it.
Has high boundaries to adoption
Some tools require large amounts of training or know how to use. They may be very technical to set up (like launch centre pro) or require the adoption of new habits or ways of thinking, (like…er launch centre pro) and so they make it difficult for someone to start using. If you aren't using an app regularly at the start then it probably won't stick around.
There are two main applications you can take out of this when you come across a new tool.
1. Will it stick?
Use the above critieria to evaluate an app on it's stickability. If it looks delightful, easy to use, adds incredible new features which nothing else has and has no flaws, you are on to a winner. Though no app Is perfect and it's only a matter of time before a better tool comes along.
2. When adapting to a new tool, pick the best time
I'm looking to switch over to dispatch this weekend. I've waited to switch my email client because I know that I have a lot of habits built around how I manage my email. If I tried to switch on any old day I'd no doubt fail and just stick with my current tool (mailbox and gmail) but by waiting for a moment where I have time to explore the features and set them up properly I am giving myself the best chance of letting it stick.
What makes a tool stick with you? What are some examples of sticky tools you can think of?