i own over 3000 apps across all apple machines (including adobe software on mac), and many android apps as well. what gets me to buy is this: the app provides me with an excellent, fully functional, experience free. it also lets me know i can have even more if i buy. it informs but doesnt nag and doesnt do sneaky self-promotion, and is mostly free of other such nuisances.
if two apps are equally good, or close, the one with the handsomer icon wins. if an icon is flatout ugly, i will always be searching for a replacement that does the same thing without make my eyes hurt.
hipstamatic is a prime example. its filters are so good you want them all. its posible to grab the app and shoot fine photo with just the default filters that come free, they regularly give new "freepaks" (that go paid after a day or so, meaning get them free quickly or buy them at your leisure) and also regularly make new paid ones. i buy every single one, as do many, many people. but i might not have even tried it had it not been free and functional as such. using it and seeing its excellence is what turned me into a buyer, ongoing.
other examples: alien blue, camera+, friendcaster, fruit ninja, talkatone, angry birds, paper by 53, twitterific...notice anything? these are all top apps. users can have a complete and excellent experience without every buying anything. if you want more, more features or more ad-free or some such, you buy. with talkatone, i finally bought simply from gratitude. its been my main calling app for over two years, and the support (seldomneeded btw) is prompt and good. with truly good apps,its hard to imagine anyone /not/ wanting more.
teasers -- for me anyway -- are basically ads. as such i tend toward skepticism and if they are frequent, they become increasingly irritating. occasionally ive permanently deleted an otherwise good app -- which i might later have upgraded to paid -- from sheer annoyance at being nagged. (i also skip trying other apps if the first one was that much of a pain.)
this, from someone who eschews adblock. my son programs professionally and i occasionally beta test. i understand that people need to live. but im guessing teasing (unless done really, really skillfully and sparingly) would yield more losses than gains. let me /experience how it is to enjoy the app./ let me use it daily until it becomes essential to what i do. then I'm hooked and will certainly upgrade. its much like....drugs.
while I'm here: dont remove features after someone's paid for your app. or if you do, please read the reviews amd if the feature is widely and loudly missed, restore it. i quit using one app because they removed the two reasons i used it: it was the fastest of its kind, and it had a stunning fliter no one else had (i do a lot of iphoneography).
and dont force users to do anything! (unless its essential for using the app at all) i quit using one app -- that had paid me to help with filters as they were growing -- because they force users to add tags to everything. i don't have time for it. if the image cant speak for itself, i as an artist have missed the mark. let those into tags tag, or code in auto tags if you insist on tags for everything, but dont try to make me do something unnecessary a thousand times over.esp when you have plenty of competitors who allow me a choice.
if you make a great app, let people actually use it so they /know/ it's great, and refrain from being sneaky, bossy, or stingey...you should be able to get many upgrades to paid.