1. is it something i can be passionate about? for me this often translates to "can this idea (executed well) make a positive impact on the world?" i.e. socially and/or environmentally responsible?
2. logical business model; i.e. is there a clear path to monetization / profit?: This is closely coupled to "can i sell it?" which is closely coupled to "is there existing demand?".
2.5. who's the buyer? I much prefer selling to larger business customers. Consumer models are less interesting.
These two filter out 9+ out 10 for me. #1 is easy and pretty binary - it's either interesting or it's not. Although the project i'm currently working on didn't really excite me at first glance, watching others get so excited about it really made an impact on me and i'm hooked! If the idea survives these two i bake it for a while, do some market research and competitive analysis to get a feel for market size and competition and potential demand. I then go talk to a few people i trust to give me straight feedback. If the idea still floats i start pitching to people whom i think are prospective customers. if they get truly excited about the idea (i.e. they are making suggestions and asking questions and are clearly interested) then i start to iterate on the above process. Part of the iteration process for me is to boil up a list of reasons why the idea won't fly.
I'm a recovering engineer and a sales guy. I love technology and solving interesting problems with technology and i hear tech startup ideas all the time. if it is not clear that demand exists then it's just not interesting. Creating demand is very hard and takes a lot of time, money and effort. If i can't quickly see a clear path to sales (that satisfy demand) then it's just not interesting.
The potential for a $ billion market cap isn't an early filter for me, but it becomes a filter if it is clear i will need significant outside investment capital to get to initial traction. These days the ideas/models that i can get to traction (paying customers) without significant outside capital are the ones that win. Raising money takes time and i'd rather spend my time getting customers than raising money. Global potential isn't a filter at all. I suppose if i was really passionate about the idea and the business model required significant capital and the US market just wasn't big enough then i suppose global would be important, but if the US market isn't big enough it likely doesn't pass filter #2 anyway.