For a start, text messages aren't arranged in conversations as is now common. Then there's the picture viewer, which as well as insisting you view it in landscape mode is slow and awkward. Email support is also rather basic and the lack of WI-Fi means you're limited to slower mobile data at all times.
Arguably, considering how basic the phone actually is, its interface is generally a bit cluttered and overly complicated as well: something you'd certainly get over, but it makes for more of a learning curve. For instance, sending a text message to multiple people takes an inordinate number of menu options and confirmations.
Thankfully Nokia hasn't slipped up when it comes to the basics as the main controls are large, responsive and generally easy to use. The keypad keys, though a tad wobbly, have a nice light action, making it easy to bash out texts nice and quickly.
Call quality isn't let down significantly on any particular front. The tone isn't quite as fulsome as the best phones, but volume and generally clarity is adequate. There's no active noise cancelling, though, so the recipient of your call will have difficulty picking your voice out if in a noisy environment.
As for battery life, this is where the merits of a basic phone really shine through. You can go for a week between charges and we've no reason to doubt the official figures of 12 hours talk time.
The Nokia C5 certainly isn't perfect with its slightly cluttered interface, lack of Wi-Fi, screen with poor viewing angles, and no support for nested text messages. But, if your main concern is getting a slim, light and well-built phone simply for making calls and sending texts, then it's still a contender.