1 - If you like watching Seagal beat up bad guys with little effort, grab your snacks and beverages to watch a bunch of those scenes per episode. If you want to see him speak clearly or show any emotional range, fuggeddaboutit.
2 - The plots are almost comically convoluted, with more double-crosses and hidden agendas than one should have to endure.
3 - Be prepared for a whole lotta stupid. Characters regularly fire far more shots without reloading than their weapons could hold. Outgunned protagonists routinely waste ammo by firing multiple shots when conserving bullets is vital. Good guys with pistols are constantly facing machine guns, and rarely bringing the weapons needed to level the playing field. After downing a foe, they frequently fail to finish obviously necessary kills, or check the fallen bodies to be sure they're no longer a threat. Or pick up their weapons for much-needed firepower.
3 - More stupidity - When time is short and thugs are all over the place, Seagal wastes time and effort by beating guys up slowly rather than dispatching them efficiently to help the rest of his team with the rest of the minions.
4 - Several members of his team, including some who are actually enjoyable to watch, die along the way. The overall story doesn't really require that - and some occur off-camera, so they don't even get the poignant death scene they deserve. That raises questions as to the reason. Availability of cheaper replacements? Fed up with Seagal's ego running everything to his tastes? Better offers for other productions? Seagal creating turnover so viewers will stay more bonded with him than with the better actors around him?
5 - This one's a biggie. Season One sets up a huge cabal of bad guys with a nuclear threat, plus a personal revenge quest up their food chain of evil for Season Two. Even with the subsequent 2012 movie/extended episode, they don't finish the tasks!!!! The thing ends as if they were planning another installment that no one was interesting in funding. Or filming. Or committing air time for. If you crave closure, spend your time on another product.