Ooh, this is going to be a good thread - so much to grumble about! I live in Germany, but I think most things (including brands, sizing scale, etc.) are the same.
The thing I hate most is that maternity wear in mainstream shops (H&M, etc.) is always in the farthest corner of the shop, usually top floor or cellar, as far away from the door as it's possible to get. And then usually also far from the changing rooms. This drives me mad - it's a hassle going shopping when you're pregnant, and just when you most need things to be made a bit more convenient for you, you find the absolute opposite.
Second annoying thing is that most big manufacturers only produce trousers in standard length, meaning you have to take them up or let them down (or, if you're like me, find a tailor to do it for you). So again, even if you find trousers that fit, the quest is not over - you have to take them, try them on again, measure the hem length you want, sew/take to tailors. etc. Some more upmarket manufacturers do produce shorter and longer trousers, but they are a lot pricier.
Thirdly, so much of the clothing is so bland! I suppose that's logical when the market is comparatively small, but still - black tops, black skirts, blue jeans, floral blouses, striped cotton t-shirts are fine for a while, but it would be nice occasionally to find something in a daring colour or with an interesting neckline...
This is a bit of a rant I know. I am just tight-fisted and I hate shopping, and so I really resent the stress and the money I've spent over three pregnancies so far!