I think a long engagement is fine. I don't get the comments about not being taken seriously as a couple, either.
While it may be bumped up, my BF (soon to be fiance) and I plan on getting married in May 2012. The best venues book at least a year in advance. We'll be able to get our first choice cake designer, caterer, band, DJ, florist... We can block out and get the best rate on hotel rooms before some convention in town books. And we get to have a fun engagement party before planning a wedding!
I think long engagements make a LOT of sense when the couple is either finishing graduate or professional degrees, starting their careers (you want to have accumulated enough vacation time for a decent honeymoon), or in the process of buying a home. Why would you want to add the stress of a wedding on top of all that?
I can understand your reasoning for deciding to have a long engagement. You've set a date and decided how much time you need to plan for that date. What I don't understand is engagements where the couple isn't planning to set a date yet at all, or they're waiting a year or two years or even more before they do. Personally, and I know this isn't how everyone thinks about it, but I got engaged to plan a wedding and start a marriage. If we weren't planning the wedding and getting ready to be married to each other in a serious and focused way, I wouldn't have gotten engaged. I think it's a bit silly when people are engaged for three plus years, and they're waiting to start planning/get married till they have more money saved, or they've bought a house together, or they've finished their degree program, not because I don't think those are good reasons to wait to get married, but because in those situations, I wonder why they're engaged at all. The couple in this situation isn't ready to be married (all those valid and good things, like saving and investing and educating) are in the way, so why even get engaged? That said, I think every situation is unique, and there very well might be a couple out there with a good reason, and I just haven't heard it yet.
I don't think there is anything wrong with a long engagement. That way you'll have plenty of time to save money and really decide what you want for your wedding. My fiance proposed in August 2009 and we aren't getting married until February 2012. By the time we get married, we'll be engaged almost 2.5 years. Plus you'll have plenty of time to enjoy being engaged!