Energy efficiency is a must and depending on your home and siding and ect will help with what styles. I like vinyl windows with j channel built into them.
Is the contractor a dedicated window installer? Would Andersen or pella hire them to install their windows? Ask to speak to a recent customer who had the contractor install windows for them!
These days it is better to discuss window details with a window rep from a supplier/dealer than a contractor unless he is a dedicated window installer. Even then, a window installer might only offer a small selection. Reps are schooled in there product(s) and usually can sniff out what suits a client’s taste/needs. If you don’t feel like a rep has your best interest in mind or isn’t helpful finding what you want, move on to the next store.
Now as for what to discuss about windows: New construction or remodel? Materials… wood, fiberglass, metal, vinyl, etc. Style… single hung, double hung, slider, casement, awning, picture, etc. Color! A lot of companies now have vinyl in a wide selection of colors. Pretty much all windows will comply with new energy codes (always check) which are a lot tougher, however if you really want performance, check out triple glazed windows (Marvin leads field.)
Last, but never least, BUDGET… always START with the budget up-front; you will save a lot of time that way.
…and when having windows installed read up on proper flashing, talk to your rep and make your installer knows the manufacturers recommendation (necessary for warranty), be sure you understand it and enforce it! If you skimp on this detail you can destroy your house.
You should know who manufactures the window you are choosing, and the model you are having installed. You should confirm the warranty information. Review the installation process and make sure it is in writing.
If you want new installation windows I would shop around with different contractors, not reps that only work for certain manufacturing companies or distributors that have deals to sell certain brands of windows in their showrooms. Contractors who work with all brands will be able to give you a fair assessment of the pros/cons and what will work with your budget. Brand specific companies just want to make the sale. Unless your windows are falling off the house and are completely rotted beyond saving, I always recommend getting a repair quote as well. There are some great companies that can keep the original windows and upgrade your glass, hardware, fix rot, reseal, weatherstrip and perform maintenance to aid in optimal operation at a fraction of the cost of replacements. If you definitely want new installations, go for Energy Star certified products to reduce utility costs and possibly receive tax credits.