For technology geeks, one of the greatest enemies of productivity is teh Shiny, i.e. some algorithm, framework, language that is a possible solution to the problem at hand, but mostly attracts because of its dazzling awesomeness. When something seems like the cool way to do it, chances are you are about to step into a big pile of YAGNI.
Often, the simplest possible solution or, even more importantly, using what you know, rather than what appears to be the "best", is the way to go.
A good example is the boolean algebra parser for my tag queries. Once I realized I had a mini-DSL on my hands, my google fingers were refreshing my knowledge of Coco/R and evaluating Irony in order to build a parser to build me my AST. It took stepping back to realize that to get this to the prototype stage, some regex and a simple state machine could handle the parsing much more quickly (mostly because it avoided the tooling learning curve). It may not be the final solution, but it's one that works well enough to serve as a place holder until the rest of the tech proves out.
Don't get me wrong, this does not mean, hack up the simplest bit of code to get the job done. A straight path to the solution is likely to result in an unmaintainable mess. Certain standards of code quality need to be met, so that you can still test the result and be able to refactor it without starting from scratch. But if you meet that criteria, getting it done fast first, buys you the time to evaluate whether a) the problem you're solving is the right problem, b) the solution provided is sufficient for the intended use and c) whether the shiny tech really would improve the implementation.
Determining whether a path is just shiny or appropriate isn't always easy. One bit of tech that's been consuming a bit more time than I wish, is basing my persistence story on NHibernate, instead of rolling my own. The temptation to use what I know is strong, but fortunately (or unfortunately?), I have enough experience from rolling my own ORM and hand-coded SQL to know that down that road lies madness. So, be sure not to mistake "learning curve" for yagni.