A quirk of the serious wargame genre is that, regardless of the era it was built in, a decent wargame won't really ever become completely obsolete. While it can be incredibly annoying to go spend $100 on a brand new sim, only to find out that it's not very different from the sim that cost you $100 a couple years earlier, it can also be really nice to bump into $8 games like Combat Mission that still hold up very nicely.
At it's core, CM is almost arcade-like in how it presents - the interface and controls are VERY simple, especially relative to it's contemporaries. This is a big part of why it's aged so well - what was a sort of sparse interface back in the day comes across as relatively streamlined to modern sensibilities.
The gameplay itself is pretty deep, despite that presentation. As is to be expected, morale, ammo, visibility, experience, everything factors in to a unit's performance, and it plays like a wargame, not like an RTS.
Likely the most noticable failing of the game is in the campaign layout. Unlike other titles available at the time, there isn't really actually much of a real linked campaign. This isn't really a pro or a con, because at it's heart CM is all about quality scenario play. While I do miss the added strategic element of having to carry (and thus preserve) a force across a long campaign, it's not a deal breaker.
The other really notable sign of it's age are, of course, the graphics. While the game does a weirdly amazing job with weapon effects (the sounds are great, the explosions feel really, really legit, and the chaos of a serious gunfight is conveyed pretty well), it's hard to ignore that the scenery is ugly as hell, and, describing them generously, the unit depictions look like characters from Goldeneye on the N64.
For $8 bucks though, if you can ignore that, you essentially have a more or less modern, softcore wargame that, provided it runs for you, competes on a gameplay level with any similar newer offering.
