my first thought: "spacing!!!" on the main page, you need spacing between the links (do it via CSS). same on the download page: paragraphs, links, individual lines (use "line-height"). they'd all be easier to read if they had some free space around them. it would make headers stand out more, too.
and yes, at least a tagline would help a lot. even just "freeware aircraft for Flight Simulator" in big letters would be good.. but put it in a H1 tag, you know, for search engines and stuff ;)
well, those are my 2 cents, hope that helped. personally, I would also make the page centered and fluid, but that's a matter of personal taste I guess, no "biggie". but the spacing, that is a biggie! (to me :P)