In no particular order:
- Why show match numbers so prominently? Are they relevant to the typical user?
- On the match listing there is a blue button that looks like it should be a dropdown that does nothing? (not logged in)
- The flag icons on the server list are pretty ugly - they are also not centered vertically within their parent element correctly
- Map names - I'm not sure if TF2Center supports custom maps, I haven't checked but listing "cp_dustbowl" as such seems a bit... technical. I'd filter it down to simply "Dustbowl" or "Well (CTF)", but that really is a minor nitpick.
- The layout seems odd to me - I'm used to designs where anything auxiallary to the main purpose of the site is placed on the right hand sidebar, but the first thing I see on TF2Center is a list of players online. The players online space also commandeers far too much horizontal space. I'd also say that, if there were extra horizontal space for the lobby listings that each listing should have around 1.5x-2x the vertical height, the map images currently seem very cramped.
- The main jumbotron strikes me as a bit odd too, it's a lot of empty space for no real reason. I'd add a call-to-action button for logging in and perhaps use a screenshot as the background instead of the radial gradient.
- The scroll bar for players online looks like an up/down switch, I actually tried to flick it.
- Class icons/occupied state on the lobby listing - There are two tiny dots on the bottom right of the class icons that indicate which teams still require that class. These are quite subtle and I'd go for something more obvious like making the background of each class icon a circle constructed of two semi-circles representing the team colour, initially these are just the outline of a circle but are filled in when someone joins that class slot. Alternatively, make the circles 3px tall bars spanning the width.
- Mumble icon could do with more contrast.
- User profiles should use vanity URLs (/id/) over the generic /profile/ if possible
- Remove the white border on the sign-in through steam button - it current has 4 levels of framing (nav border -> nav background colour -> white border -> image border)
- None of the content in the nav bar is centered vertically.
- Don't offer downloads of stock maps.
- Class icons throughout the site need alt-text/tooltips - especially those for unique roles like pocket soldier
- On the setup page for a lobby there's an image of the map to be played in