From memory, Lille-Poitiers goes from Lille Europe rather than Lille Flandres (I could be wrong here) - but check if it's not explicit. Lille Europe was built specifically for the high speed line (that connects the Channel Tunnel to the Paris line), and is minimal, but acceptable. There's a couple of sandwicheries (your standard Pains ŕ la Ligne places) and (what else?) an Irish pub.

Check your reservation. If you're not happy with it, there are 'échange minute' machines throughout the station, which allow you to change it free of charge (assuming you have an SNCF-issued ticket). Very useful - it's definitely worth going for the right kind of seat (individual, if you're travelling alone).

Not all TGVs have catering, even in 1st class. This particularly applies to the cross-country ones. The last one I was on, from Lille to Chessy (which I think was headed on to Bordeaux), had nothing but vending machines. If you're expecting to eat, check up on the schedule, and get something at the station, if necessary.

1st class is comfortable enough, but some of the older carriages do look a little worn now. The old-style 1st class (where the individual seats are) do not have seats that recline. The 'club duo/club quatre' seats do, I think. There's also a 'salon' at the end of the train, which can be booked for groups, but is very often empty (the cross-country TGVs can be awfully quiet).

Otherwise... Enjoy your journey! I always like travelling on these machines: the speed at which you see everything go past is impressive. The traffic on the Autoroute du Nord seems stationary as you go past (and not just on the run-up to Paris where it usually is stationary...), and you can see the scenery completely changing as you glance up from your book, such is the speed... Look out for the inevitable change in weather as you pass Senlis into Ile-de-France