On the Philadelphia Phillies roster project: I began the Phillies roster project in May 2010, and it was completed in December 2011, so the project as a whole took me 19 months to finish. Originally, there were 21 articles in the topic, but during the course of the various and sundry FLC nominations, it was reduced to 18 through mergers. The most time-consuming part was definitely the table building. That was the first part of the project that I tackled and it turned out to be a smart move. If I hadn't had all the tables done, it would have been nearly impossible to write the ledes for each list and I probably would have quit somewhere about ... probably letter F or so.
The most difficult part of this project was adjusting to the changing FL standards. At the time that I nominated Philadelphia Phillies all-time roster (A) for featured list, a site-wide movement for better compliance with Wikipedia's policies on accessibility was gaining ground at FLC. I offered to make that first list a testcase for successfully implementing the improved accessibility standards, but this occurred after the lists had already been completed in userspace and moved to the mainspace. So I had to go through and change the format of each list. (I have to give a shout-out to RexxS for his help on the ACCESS requirements.)
I'd be remiss if I didn't mention the most fun part: all of these lists were moved to the mainspace at the same time, and they became a gigantic DYK. I had a lot of good times developing a good hook for that.
I chose to improve the Phillies' roster lists because it is an intersection of my two big loves on Wikipedia: the Philadelphia Phillies, a team I've followed since I was a young child; and writing featured lists, which gives me great satisfaction. I really do enjoy taking on large list-related projects. Some previous baseball-related list projects of mine include the series of Rawlings Gold Glove Award and Silver Slugger Award lists and several lists of Major League Baseball managers.
On KV5's plans for the future: I haven't quite decided on my next project yet, but rest assured, I'll try to make it something monumental! I have started improving the List of Philadelphia Phillies owners and executives, using a book I received as a gift last Christmas (yes, I asked for a reference book for Christmas just for Wikipedia), but I've gotten away from it so maybe I'll tackle that next.
On writing featured lists: Featured lists require significantly less prose, but there is a lot of specialized mark-up that writers need to know to comply with accessibility standards. For new writers trying their hands at FLC, I always suggest peer review before an FLC nomination. There are so many little things here and there that can hang up an FLC nomination. Always check and double-check the reliability of your sources. We have the reliable source noticeboard or the FLC nomination talk page if you have questions on those, or you can seek out help at an appropriate WikiProject talk page. Finally, always review the appropriate criteria and make sure that, in the case of featured lists, your list meets the standards for prose, the lead, comprehensiveness, structure, style, and stability.